You can join communities during Perch sign up. You can also join communities from your groups community management page.
From the main menu, navigate to Intelligence->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.