Check some other Defense or Prot Spec threads as they might help you. If your group knows how to play their class respecively, then there shouldn't be a problem. Just let the group know not to start in the DPS until you have sundered, thunder clapped, Demo Shout and so on.
The Mage has really good Burst DPS and will/can pull aggro easily, so tell him not to try to crit or blast everything in sight to reach new high's. The priest usually doesn't grab aggro too much even when crit healing or AoE healing for that matter, that I have seen. I happens, just not often.
All I can say without repeating what everyone else says, is to read the threads on tanking, and not just the FAQ Sticky. All in all, your buddies need to know how to play their class and need to let you tank. You're only as good as your group/healer lets you. IMO