Using Trinity you can bind the bar targets to 'party1' , 'party2' etc.

This caused a little confusion at first but the simple workaround was - always have the same char do the group invites, and always accept the invites in a particular order.

My slave 1 issues the invites, and I accept in the order - Main, slave 3 , slave 2, - this way my party is always set up in the same order. I guess if you were grouping with someone else you would invite them last and they would always be party4.