Fellow in the broader sense does mean that but in several translations it says "with me" And you are right, it is not solely on Junia's case but it is a wider thing.
I think we are a bit presumptuous in assuming a well defined structure existed in the early stages of the church, and I'm confident that the labels we now use interchangeably were not in use then. I recall a similar debate among scholars with the name "Theophilus"......a person, type of person, or group of persons? All fodder and job security for Seminary Professors and Theologians! Jeff+