Hi Tom, while I can’t speak for everyone else, I can tell you that I had to contact Medium Staff’s help desk to get memberships manually turned on for my publication. They’re fairly responsive and helpful.
You’re correct that the membership program was beta-tested with only a handful of established publications, but based on my research, all types and sizes of pubs can have memberships now and the number of followers shouldn’t be a limiting factor. It’s probably only a matter of asking. Best of luck. Let me know if I can help.