Crossing District Boundaries

If a party leader accepts an invitation from a party in another district, the smaller party travels to the larger party's district, regardless of who invited whom. The party leader of the larger group becomes the party leader of the new group. The smaller party sees a countdown before switching districts; during that time, their spots are reserved in the new party.

If both parties are the same size, the party that sent the invitation is treated as the "larger party". The party that accepted the invitation switches districts. If there's still room in the party, the new party's leader can continue to add henchmen and invite additional parties. http://www.guildwars.com/gameplay/gwnfeatures/partysearch.php