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 Recommendations widget is a useful way to find communities that you might be interested in joining. The widget displays in the following views: I'm an Owner, I'm a Member, I'm Following, I'm Invited, and Public Communities.