I was wondering when you would speak Ellie :-)
I think it is unecessary to limit the amount of fellowships a player can join, or how many members a fellowship can have. Players ulimately will only play in as many fellowships/ teams/tournaments/games as they have time for.
There are loads of clubs at goldtoken, but I believe when the clubs first started there was only a few big ones, which divided into more smaller ones, and so on. Ellie can fill us in on the details as she has been involved there from the early days.
At present the big fellowships have pretty much invited all the members here, and a lot of people have joined 3 or 4 fellowships, without knowing quite what to expect. If this continues then of course you are going to get confusion, and people won't know where their loyalties should lie. Arguments could quite easily arise out of all this. I know at goldtoken, to join most of the clubs you actually have to ask the president, invites aren't automatically sent out, thus you have players who do a bit of research for who they wish to play for, they might choose a club because they have friends there, because they play lots of challenges, they have good players, lots of reasons, but usually the choice is left up to them.
I put it this way, if and when Fencer decides to implement inter fellowship tournaments it will be much more fun for us all if there are more than 4 or 5 fellowships, more fellowships with smaller teams, will give us much better competition in the long run, and as more players join and select their fellowships then more friendships will be formed, players will also move around a bit, and fellowships will grow, players will leave and start their own, and so on.
Also as we have seen at Goldtoken there are clubs which have been formed for the purpose of tournaments only within that club, members can belong to any other club they like, I am co-prez of one there with 150 members which I formed to only play backgammon tournaments. There are also clubs formed for the purpose of sharing hobbies and the like, these clubs do not compete in outside tournaments either, so any member can join and not affect their team standing with their main club. There are lots of possibilites

The way we deal with which players actually play in inter club challenges there, is this:
A club may post for a challenge, or contact the president (big boss) directly. The will specify how many players they have, and the challenge details, ie. 3 day games, 2 rounds, (depends on the formats we get here)
Then the president will post on the club board and describes the challenge, how many players are required, and asks who wishes to play. If there are far more who wish to play than there are spots for, then we go find another challenge for them, another reason why the more fellowships which develop here the better.
Hope this helps :-)