Mixed Groups: Interoperability Between IGMPv1, v2, and v3
It all boils down to the least common denominator. When there is a mix of hosts on a subnet, the only method for them to communicate is with the lowest IGMP version number used by a host. This is due to the fact that higher versions understand lower versions for backward compatibility.
The other situation you may encounter is having a mix of clients and several routers on a subnet. There is no mechanism for routers configured with a lower IGMP version to detect a router running a higher version IGMP. This requires manual intervention, and someone must configure the routers to use the same version.