You could assign ELO at the group level. It will increase the number of possible groups by C(N, g) where N is the number of players and g is the group size. It

You could assign ELO at the group level. It will increase the number of possible groups by C(N, g) where N is the number of players and g is the group size. It could work if the groups are stable enough. If you have individual ELOs, you could pre-seed the ranking using averages and then let the algorithm take over.