Re: Users are not being added to the Main Blog?
If you have a backup of your files and DB, you can always revert, but I guess it’s too late for that.
Anyway, because this sounds like a WPMU problem, you might want to head on over to the WPMU forums and explain your case there:
https://mu.wordpress.org/forums
How did you make the move to WPMU? That could be the problem.
Also does this problem occur if you use the default BP theme?