I'm thinking not, because of the premade nature of the group. You can change your class, like in pvp, but probably not summon a companion.

Which reminds me, what happened to the Grand Company content that they talked about being in 3.1? There were supposed to be new GC ranks, and something about NPC mercenaries, I thought, which suggested additional companion options? There were a handful of things that didn't get any discussion in live letters, making me think the content has all been cut.