The app's new home page will have Gatherings galore! If you want to attend a Gathering, simply click on it! (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!)
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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 click on a gathering and press the “ATTEND” button at the bottom of the screen. You’ll then see the button change to show you are now “ATTENDING” the Gathering!