What Class Does This Group Need?

[JurisDictum]

Rogue or Mage would be the best. With proper gear -- Rogue will out dps anything except certain charms. Rogue's are also able to help CR a group with hide/sneak and open doors that are really annoying to get keys for.

But on low gear, Mage deals a lot of damage. They also have mod rods, pet weapons and the ability to summon people eventually.

Ranger is a possibility too. Tracking can be useful and extra dps is always nice. They get a few cool buffs late in the game as well. But I think a Rogue or a Mage would add the most.