違う都市のグランドカンパニーに入団している人とでも、グランドカンパニークエストを行うことができます。
またPT内に3都市すべてのグランドカンパニー所属メンバーがいると、いくつかのクエストでいいことがあるかもしれません。
You can still do the Grand Company quests with those from other Grand Company nations. When in party if there are members from all 3 nations (3 people from different nations) with some quests there may be good things.
From Reinheart's translation thread. Caravans might be restricted but everything else will not be.

Also BC's in FF11 you could only do with people that were on that mission or had done that mission previously, so it wasn't all good.