I'm actually having the EXACT SAME PROBLEM! I invited several people to be followers in my blog, but only because I didn't realize that changing them to be contributors later on was not an option.
One of the people I invited accepted my invitation to be a follower, but when I tried to change his role to be an author, I just couldn't. He doesn't appear in the "All Users" section where you're supposed to be able to change roles! The one place where he does appear (the "Site Stats" place) does not have any method for changing roles.
I sent a new invitation to everyone I had invited, this time asking them to be Authors, and it worked! The catch is, it only worked for people who hadn't already accepted my invitation to be a follower. For this person, it just said "User already has a role."
I think something must be wrong . . . seriously wrong.