Anybody with a Stack Overflow account can join a Collective!

Visit the Collective's page (by clicking on the Collective's badge on a question page) and press the "Join collective" button. The Collective will now show up in the left sidebar.

Alternately, you can join a Collective by clicking the "Explore Collectives" link in the left sidebar, and then clicking the "Join" button next to a Collective listed on that page.