You can join communities during Perch sign up. You can also join communities from your groups community management page.
Click the gear icon settings in the menu and choose Communities.
From there the group community management page, you just need to click on the Join Communities icon add_circle near the top right.
From the modal that pops up, you can easily join any communities.
After you click Join next to a community, you may be asked to select which feeds you would like to subscribe to.