Hello.

This is a suggestion and also a query.

How to allow guests to use search on our board ?

It serves also to see All Activity.

Thank you.

Guests can use Search by default, but there are a few settings which would disallow it (or restrict how often they can use it) if you've changed those settings. 

And they also, by default, have access to the "All Activity" Stream.

If that is not the case on your site, please review the system settings in the Guest Group, and make sure the search module in the System app (ACP -> Applications) is enabled for Guests.

Great. It was indeed disallowed.

Thank you.