I am having the same trouble.
I have 2 WordPress blogs set to private with different URLs of course.
A user joined the first blog fine. Now he wants to join the second. When I "add user from community", I get the error message "User not added, User already registered."
I have activated other people who have joined both blogs in the past without a problem.
It's kind of an urgent problem.
The first blog is http://ttheoryforum.wordpress.com/
The second blog is http://ttheorypro.wordpress.com/
The user e-mail in question is [email redacted]
Like I said, he joined the Forum. Now we need to activate him to Pro.
Many thanks for any help.