Joining a community. What's it all about? It's usually more formally called an "intentional community"; but what does that mean? Let's roll this back to the more general, the inherent need for basic community, for all human beings (if indeed it is a need, a required-necessity).
Community also semantically seems to be a word comprised of two words: common and unity. What do we have in common which unifies us as a group?
The academic etymology would be important to touch upon. Let's start with the popular wikipedia source, although not to rest on its purported authority. "A community is a small or large social unit (a group of living things) who have something in common, such as norms, religion, values, or identity. (and) ...a community is a group of people living near one another who interact socially."