How do I create or join a collaboration?

Scratch doesn't really have a feature to collaborate on projects because it could lead to private communication that isn't monitored and they don't want that. Currently the only way to collaborate on a project is by remixing each other's projects. Sorry!

You can go to the collaborations section of the forums and maybe ask if you can join/create one

You could make a project, ask someone to remix it and give them time to remix, unshare it, repeat with other persons project and so forth.