If the event gets updated, do people who have RSVP'ed get notified? If not, can I make this as a suggestion?

I agree. RSVP as attend should also mean follow.

Follow is a separate option, but the suggestion here is an interesting idea. I'm not sure if you'd want to automatically follow it, but you should probably definitely get notified if the date is changed for some reason.

Where is this RSVP feature?

Where is this RSVP feature?

In the Calendar. :)

Is it like an Easter Egg hunt? I don't see it in the calendar.

It's disabled by default in the ACP. It's one of the individual calendar settings (as in each calendar you make can have them enabled or disabled).