I'm writing some scripts to synchronize groups between a few different services, and I noticed that there was no RhodeCode API for removing users from a group.
I've made this addition in my fork. It's pretty simple to add, but I'll send you a pull request if you want it. If so, would you prefer it on the default or beta branch? I've added it in both, but I am currently unable to run on the beta branch so I haven't tested it there.