Depends on what you class as a group. It is unlikely that their unfederated server will come close to the number of users on even the 114 public federated servers, much less all the federated private servers.

I noticed something interesting while looking; it appears that at least one public XMPP server has experienced a recent large influx of new users.

>"The number of daily active users has increased more then three-fold in the last year alone." [1]

[1] https://jabber.at/b/temporary-suspension-of-registration/