It's important to remember the purpose of groups in the first place - it's for people who aren't a part of any town or allegiance except to those in their groups. It's for bandits, mercenaries, traders, and diplomats travelling in from other realms. It's for people who don't have a town guard, in a way, or - are their own town guard, if you want to see it that way. They're on their own. So in a way they're like opposite to guards. Group types allow us to keep people from just constantly grouping up to defend each other all the time in counter attacks because you can only be a part of one group at a time. Military: leaders direct attacks, gossip: group chat, trading: multi-inventory marketplace. Military groups should be able to have a designated co-leader so that the group doesn't fall apart as soon as the leader dies.
Group only discussion - eavesdroppers? Leaders can give orders? Should there be group types to allow for certain types of actions in a group? For example, a military group leader can give orders to its group members (even while they sleep). A trading group? A gossip group (everyone can chat together, but not publicly)? A travel group (people only follow the leader)? No one else will know what type of group it is unless orders are given that are visible and give it away?