See also the official documentation:
Help Center > Members Manual > Working with calendars > Using the RSVP feature

How RSVPs Work

[NOTE: The RSVP function is only available in Premium and Enterprise groups.]

Here is a summary:

• Events can be added to a calendar and an RSVP requested. See https://groups.io/static/pricing#what-support-for-events-is-there 
• An RSVP link will appear in the automated emails of type #cal-invite and #cal-reminder. Clicking this link will take members to the event RSVP page.
• If members are not logged in, they will have to do that first. Again: members must be logged in to RSVP.
• An event can be limited to a certain number of people, including guests. If members RSVP within the limit they get confirmation that they are on the event.
• If members wish to attend and the event is already full, they are added to a wait list. There is a message shown at the top of the screen to confirm this.
• A member placed on the waitlist is told their position on the list when they are added. There is no way of them checking to see if they have moved up the list once they are on it.
• There is an option for the event organiser to email all attendees, download the list of attendees, and/or print name tags. 
• If an attendee updates their RSVP status to say they will no longer attend, the first person on the waitlist is automatically added and they receive an email to confirm this.
• New premium feature, Send Invites On Join, sends an ICS file of all calendar invites when someone joins a group, and sends an ICS file of cancel notices when someone leaves a group. 2020-02-01

It has been found that event waitlists cannot be managed manually by the organiser. So, for example, if an event organiser knows that a previously confirmed attendee can no longer attend but they are either unable or unwilling to update their RSVP status to free up a space for someone else, the organiser cannot do it for them. The only way of adding the next person on the waitlist for the event is by adding another space.

Below is an example of an RSVP screen. Not everything was populated when this event was set up but it provides a general idea.

Last updated: 09APR2020
Ref: https://groups.io/g/GroupManagersForum/message/14934 and https://groups.io/g/GroupManagersForum/message/14937