I like the idea vonFiedler, but I am confused about the groups a little bit.
Being in a group provides protection from outside forces
Explain this a bit more in detail. From what I am reading, your win condition doesn't change upon entering a group, and people in your group can still kill you, so other than this "outside force protection", I do not see a benefit of grouping with someone. Perhaps making it so your win condition becomes "your faction is the last alive", although that seems like it would drastically change gameplay.