MUC sends messages only to online users. The only 'persistance' is the chat history.
Openfire supports also 'server side groups' or you can create client-side groups in the roster.A broadcast by the client to all group membes should work fine.
There's also the broadcast plugin ...
The more important question:
Who should be able to manage the 'chat group'? Can one search for it? Who can invite/kick others? How long does it live?