Running: Be very attentive not to lose members on the way. Stop a few seconds after difficult running spots to have everyone catch up. Some people with slow comps get unstick when running uphills or after crossing water swimming. Avoid swimming and running straight uphills, avoid getting aggro and avoid causing falling damage, it weakens then group for an ambush. Always watch chat and if you happen to loose anyone, immediately turn back to pick them up. Stop in safe spots at regular intervals for rebuffs.
Watching: You can't have your eyes everywhere, you'll need to distribute watch duties. Have someone of the group watch the back, someone left, someone right. Have them immediately report in group chat if they see enemies (INC), or other problems (aggro, unstick). Having them watch for each other allows you concentrate on the overall plan. Tell them what you expect them to do, and inform them of your intended tactics so they can follow it without guessing :)
Fighting: Once you encounter enemies - there is *your* responsibility. Estimate your group power and decide whether to attack or to turn and run. If your group members don't know you very well yet, you'll need to tell or show them when to charge
Resting: Never stay in the fighting spot to rest. Most probably the enemies will report the fight and send for reinforcements. Gather your group quickly and move away to rest somewhere else.
Watching: Keep your eyes open and your attention at the task. Check for falling back groupmembers, for aggro, for enemies, but *never* act on your own. Report in Group Chat and let your Leader decide the course of action. Especially try not to unstick unless the leader decides to enter the fight. He is the one to estimate if the group will turn and run or stand and fight. If he's just luring or watching enemy numbers you'll ruin group tactics if you unstick, and most probably will also cause the whole groups death.
Fighting: In big fights, try to stay in range of your group or return to it asap. You can't expect the healers to run after you and you won't be able to help fellow group members if they're too far away. Once a fight has been starting, don't try to run, its useless anyway. It will split up your group and they will be farmed one after another from behind. Even if you get away you'll be alone and lost in enemy area and won't come far. Stand by your group, be victorious together or die together. You don't loose more then a few minutes by deaths, but you have much better odds to survive if the group works together.
As a tank you will most probably be at the front of the charge, so it is very likely you'll be among the first ones mezzed. If you are, then keep patient. Wait until the enemy casters blow out their (usually mezzing) spells and until the enemy tanks run behind you heading for your casters in the back. Use the time to find out where their healers and mages are. Target one of their casters (or /assist your main tank, since that's probably what he'll do anyway), then use Purge, *stick* to your enemy and hit. Stay stick to your enemy during the infight, and use sprint to follow if he tries to get away. Don't rest between targets, rely on your healers and stand your ground.