Joel R

IPS likes to hide little functions and tools where they think we might need them.   😀

Most people use it to give expanded permissions or features to members who may be members of multiple usergroups. 
Example 1: Targeted Permissions
For example, if you have members who monitor the Pages wiki or who write Pages articles, but they may be spread across multiple usergroups, then you can assign them special editing privileges in Pages as a secondary group permission while still retaining their primary group.  (For example, maybe they're a "Flame" or "Ember" but you only want to give trusted people access to writing articles in your community, not all Flames or Embers.)
Example 2: Overlapping Permissions
You can also sell overlapping products in Commerce to decorate the user postbit, signature, or profile.  For example, one product might upgrade the name to a sparkling name.  One product might give a special badge on the profile.  One product might allow them private albums and blogs.  Because you want to allow users to pick-and-choose between these user upgrades, you want to only add the secondary membergroup for the feature that they purchased.  
 
To be totally honest though, my personal 'best practice' suggestion is that you want to try to keep your groups as simple and as flat as possible.  Multiple membergroups leads to a complex hierarchy, which can realistically lead to permission issues.  I've seen (and been a member of) many boards where their permissions were totally messed up, because the owner never checked all of the overlapping restrictions.  You really want to audit your permissions, menus, visitor experience, etc. on an annual basis and make sure they're manageable.

I prefer to use secondary groups as informational (for me) or permissive. Basically I just like to give people "more" things with secondary groups (especially since you can't take things away) @breatheheavy
For example I have a secondary group called "new people" that members are in for 1 week from the time they join. This shows them specific "new people" widgets/blocks to help get them integrated into the site. For example it links to our welcome guide, our welcome forum, etc. After a week, the group is removed and the blocks disappear too. (I think I remove the group if they post a lot too but don't quote me I have complicated group promotion rules). I also have a secondary group that I use to see how many of my members are inactive.
Alternatively, I use a secondary group to give access to special forums on my site.
Lastly, I have Post before register enabled but there are some things I didn't want guests to be able to post to that I did want new registered members to be able to post to. So I have a secondary group for new members that provides those permissions as a secondary group (to post to these things) where the main group does not have those.
So, I agree with @Joel R Keep it simple when it comes to groups or if you don't at least verify your permissions otherwise people may be able to see things that they shouldn't be able to see.

Most people use it to give expanded permissions or features to members who may be members of multiple usergroups. 
Example 1: Targeted Permissions
For example, if you have members who monitor the Pages wiki or who write Pages articles, but they may be spread across multiple usergroups, then you can assign them special editing privileges in Pages as a secondary group permission while still retaining their primary group.  (For example, maybe they're a "Flame" or "Ember" but you only want to give trusted people access to writing articles in your community, not all Flames or Embers.)
Example 2: Overlapping Permissions
You can also sell overlapping products in Commerce to decorate the user postbit, signature, or profile.  For example, one product might upgrade the name to a sparkling name.  One product might give a special badge on the profile.  One product might allow them private albums and blogs.  Because you want to allow users to pick-and-choose between these user upgrades, you want to only add the secondary membergroup for the feature that they purchased.  
 
To be totally honest though, my personal 'best practice' suggestion is that you want to try to keep your groups as simple and as flat as possible.  Multiple membergroups leads to a complex hierarchy, which can realistically lead to permission issues.  I've seen (and been a member of) many boards where their permissions were totally messed up, because the owner never checked all of the overlapping restrictions.  You really want to audit your permissions, menus, visitor experience, etc. on an annual basis and make sure they're manageable.

When any of the selected triggers occur the campaign will be assigned.
Yes, you visit the user's profile in the AdminCP, then scroll down and look at the bottom left hand corner of their profile. You'll see which campaigns the member is a part of, and be able to add the member to new campaigns, cancel existing campaigns, or restart the campaign.
If you have a user on "Campaign A", they cannot be added to "Campaign A" again. You can cancel the campaign if you wish to stop it, or restart the campaign if you wish for the user to start over for some reason. In practice, you would typically only want to send a series of emails to a user once however, so this shouldn't be a major limiting factor in real world use.
If you disable the campaign it will be stopped for all users (however their progress will be remembered in case you wish to reenable the campaign later).

If you move the user into a primary group called Premium, then no those group promotion rules won't apply.
The software will also ask, when you set up your Subscriber settings, if you'd like to move the member back to their original group once the subscription expires.

There shouldn't be any negative performance impact on high traffic boards, no.
A campaign can be initiated by current activity, but the emails are sent via a task.

No. Pages record image thumbnail settings. They are in the settings for the Pages database. 

Huge price drop this week because of Corona craziness...
 
get it while it’s reduced and before the price increases because of 4.5 ( existing customers aren’t affected by this ) 

Sure, those things should be doable and I'll take a look. Might as well include "first post" as well in that list.

7.  Connections page - I like "People You May Know" more than the random and new member suggestions, it's much more useful.  

Just curious, how will you suggest members on "Shared interests?"  
 
Suggestions:
1. I'd love to see a block that can be both inline to posts (eg. similar to Facebook as you scroll; after X posts) and a normal block for the hot zones.  The one that's inline to posts / gallery images / comments is especially important for mobile.  
2.  Notification for other friends online (eg. "You have 5 friends online including bfarber, Daniel F, and three more").

there is a group mention plugin in marketplace
 

You can check with @Makoto to see if he can incorporate into Radical Tags, which is the de facto tag & prefix app in the Marketplace.  

You can check with @Makoto to see if he can incorporate into Radical Tags, which is the de facto tag & prefix app in the Marketplace.  

Invision doesn't offer a Marketplace app.  Do you mean Downloads (which is the real app, renamed to Marketplace for the purposes of this site)?  The Downloads app is designed for digital assets. 
If you're looking to add a grocery / breakfast / lunch / dinner menu, I would encourage you to stay with Commerce and use normal products.    
 

On search page on Recent Searches block
That will be nice if you can do here 
 

FeveBee wrote an article about this on Sunday 19 Apr 2020 with several useful pointers: https://www.feverbee.com/launching-groups/
 
The same thinking applies to Blogs.  Do you have a need for Blogs?  If not, then don't worry about it.  

♟️

Yes, you can always edit any language strings in ACP > Customizations.  
With that said, if your subscriptions don't auto-renew, have you thought about switching to Products?  They might be a better fit for your purpose, and they offer more options.  

You may also want to make edits in the friendly URL settings if you'd like to remove /subscriptions from the URL. Translations don't impact those.

"A different article style" by @TAMAN
I'd like to point out to any IPS representative who might read this topic that v4 has been out for one year, and yet the community has only managed to offer 1 community guide and 1 Marketplace purchase.  There's clearly not enough community support or community innovation for what should be the most extensible app out of the suite, and that creates a steep learning curve for new admins who are uninitiated to Pages.  I think even a handful of article block templates (like 4 - 6 different styles) can go a long ways towards helping non-technical admins put together some amazing pages.
 

You can check out 'Advanced Tags & Prefixes' in the Marketplace (although the app might be unsupported sinec the original author is no longer here, so be cautious before buying!).  
It will allow you to color your prefixes.  If you'd like images, it's probably doable as a custom plugin.  You can ask in the Customization Requests board.  

What you aree looking for is a build in function called "Prefixes".

You're misunderstanding my suggestion. Yeah you definitely don't want to delete each member one by one. I'm suggesting you use the "mass" tools available for managing accounts.
Do this:
Log in to the AdminCP Visit Members > Groups Click the number next to the "Members" group. You will see a list of members. Then click the link to prune all of the results found as seen in this screenshot
 
Repeat those steps for each group, except for admins which you'll delete one by one because you'll need to preserve your own account (there shouldn't be that many admins so this shouldn't be a problem).