Hey MichelD, I just had the same problem which may have been the same problem as yours so I'll share, even though I'm a little embarrassed. I got the "cannot add new member" warning as well. What I was doing was underneath the initiator bar (which showed my name) I added my own FWID, last name and initial, thinking I was initiating the shared hunt. However, seeing as I was already logged into my account, that info was already registered and I was to add other members at that point, not my own information, DUH.
After I figured out that little gem, everything worked like a charm.
I hope your problem was as simple as me, I mean mine.
Montec