Which is why we need better support for global guilds. I planned to do this with a new gang system on Era using jerret's global guild stuff he shared in the code gallery, but a built-in isGlobalGuild()/other functions would be ideal.
Being able to manage your global guild in-game would be a huge step in the right direction imo.
Wouldn't it be simple to just check if the global guild exist and if so check to see if they player trying to form/join the party is in it?