Shifts: Member could not be added

Hello, 

I am trying to model our team using Shifts - tied to a Team where we have 60+ users all included

I am creating several groups of machines to run(about 22 machines/groups). Some members of these groups are duplicated in others, as they are cross trained. Worked fine adding people, until I got an error after the 6th or so group, saying I couldn't add a member: 

However, in the 8th group or so, after trying this group, I could add the same member into group 8 that I couldn't in the above group. It seemed like only certain groups had been an issue? Is there a limit to how many groups someone could be in? I Tried other members who hadn't been in any groups yet and they couldn't be added either. Maybe this is a bug? 

Thanks! 

Alex