Ironically, I am also looking for the answer of same question
Actually neither MUC,nor RosterGroups can solve this problem. We need to customize them as per our needs.
May be this will help you
Persistent XMPP MUC (XEP-45), like WhatsApp groupchats - Stack Overflow
Moreover RosterGroups creates groups at client end, for benefit of client it groups contacts. Sending message to this group will send message individually to all its members.