I am having a similar problem, in that someone I invited to a group has not been able to get in, even when using the link. He has the Diigo toolbar, but cannot get in the group. He is using a Mac, and we wondered if that is part of the problem. Does it matter what browser you are using?

It's called Write to Learn http://groups.diigo.com/group/write-to-learn

We did finally get everyone into the group. He changed to a different Mac (a laptop) in his house, and that one worked...so it might be a problem with his computer.