Which class would best fill this last group spot?

[Tenlaar]

If you're looking at non-casters I'd say bard, hands down. It would fit in whether you're doing the typical tank and spank or you're doing a pet group because the warrior isn't there, the regen is huge enough to really lessen the pressure on the cleric, a second CCer and eventually charm DPS, selos helps a ton with travel, etc.