Open the Events tab. If you want to attend an event or gathering, simply tap it and then RSVP or Express Interest! (BONUS: If that Gathering is part of Group that you haven't joined yet, your RSVP to that Gathering will automatically join you into that Group!)
You can also RSVP for a Gathering from within a Group:
Once you join a Group, you have access to all the Gatherings that are happening within it! Simply tap the gathering and RSVP or Express Interest. You’ll then see the button change to show you are now “ATTENDING” the Gathering!