Currently, we limit Milo group size to 8 people, which covers the needs of the vast majority of our users. The actual max size is larger, but we simply don't like making promises until we've thoroughly tested Milos in lots of environments. If your group is very large, the worst that'll happen is that you might talk over each other.
The Preview release of MiloNet 2.0 supports groups of 4 in the US and groups of 2 in the EU. When larger groups are formed, Milo automatically reverts to standard operation (2.4GHz with Adaptive Volume support). Sub-G will support larger groups in an upcoming update