You can join a community in a number of different ways.
When a community has public access, anyone can join by opening the community and clicking Join this Community
. If the community has moderated access, you need to request membership. You can also be invited to join a community, if someone you know already belongs to the community and invites you to become a member.
The Recommended Communities widget is a useful way to find communities that you might be interested in joining.
Parent topic: Communities overviews, how-tos, and FAQs