if by "limit" you mean "maximum number of members" or "maximum number of neighbors", the answer is "no".
if you mean "can i limit who can join a group?", then yes, by ensuring that you only give the groupspec to those you want to join the group.
Michael, thank you for fast reply! By limit i mean "can i limit who can join a group", and I think I have understand your idea
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the groupspec is a capability to join the group. you can make it hard/impossible to guess by making its name contain a long cryptographically pseudorandom (sub) string or by using GroupSpecifier.makeUnique() (which does approximately the same thing). you generate the groupspec in one place, and only give it out (over a secure channel, like HTTPS) to users who are authorized according to your application logic.