Not sure if this has been suggested before, apologies if it has.
Just had a thought. This assumes that one day conquest is possible, or at least raids to steal gear from tribes villages, therefore some form of defense would be necessary. Stating the bleeding obvious, not everyone can be online all the time.
The obvious answer is for NPC guards. But then, how do you determine how many of them a group gets?
An option for NPC guards could be to make the number of them available be based on the number of members the tribe has that aren't logged in at that particular point. The guards wouldn't necessarily reflect the actual player with their stats and equipment, but a standard guard character that patrols the camp.
A side benefit would be to add some realisim and life to the landscape to see NPC's wandering the towns rather than have a village be lifeless if no one is online.