Well, I would say that the Royal Guard putting a group together would be the best way to get [i]most[/i] of them together.  It's nice and neat, and doesn't require every character to have a different reason for being in the group.  

Rolf would make sense to summon, he's one of what must be just a handful of necromancers who are in good standing in the kingdom.  Iestyn is, as you said, likely already working for the Royal Guard.  We won't know about Ved until his bio gets finished.