They would have to change one of two things:
1.) The way groups are formed and allow cross server grouping (ie. invite a friend from another server into your party). -- This would require a basic re-write on how the servers work.
2.) Change the way the PF works by entering an instance FIRST then start inviting people into party. -- This would make it so once the PF started advertising you would be locked into doing nothing until it starts up.