My Rift team is a warrior tank , cleric dps/heal, Chloro mage and two bards, but you'll definitely need a real tank to do instances since the instances in Rift are much harder than wow.
(either a warrior tank, cleric tank or a rogue tank)

The first instance on the Defiant side - Iron Tombs, for example is meant for levels 17 - 19, but if you go there with a group under 20, you'll end up wiping a lot. I had trouble clearing it when my team was in their lower 20's.

Instances in Rift are not a Wailing Caverns type easy-mode and most if not all boss mobs have insane amount of hit points for their level.