Groups are an Openfire proprietary concept. In its essence, it’s a collection of users. I’m assuming here that all players are users of the local Openfire instance.
Without knowing exactly how messages are going to be structured, it’s hard to specify exactly what you need to build. I’m guessing that you’ll build some sort of combination of a mechanism that turns a one-to-one chat in a multi-user chat, with maybe an Openfire PacketInterceptor to add some server-sided magic.
Using multiple XMPP domains voids the approach that I outlined completely. Also: I’ve given this two, maybe three minutes of thought. Before you start implementing anything, you should hash things out further!
Is it possible to create groups on multiple OF servers in various domains, make them queryable by an IQ stanza, and then allow a user at one domain to message the group at another?