Is registering groups on the main server necessary for security?

[ric2b]

I talked with David and we agreed that it's not necessary for the project for clients to go get the group server address from the main server, as it's more work to do.

However, isn't it necessary for the main server to know some information so that he can limit what users are accepted into that group with some kind of blind signature from the group server?

[ric2b]

It appears so, in this case Bob dodges his debt with Alice by creating an identity on the main server, C*, that isn't registered on the group server.