Example

not server("thrall")

Would filter out all groups where the group leader is on Thrall. Not sure if each group member's server is exposed in a way that could be filtered but if that's possible it would be even better.

This is already possible using not leader:find("doomhammer") for filtering group leaders from Doomhammer or not myrealm to filter players from your own realm. This works because the player names have the realm as a suffix except when they are from your own realm.