Its not too bad if you organize, Ive had some trouble but we learned what to do. Take 1 group at a time. CC one of the adds, several classes have a good ability to CC, and keep it from acting as long as it doesnt take damage. yes, my dumb *** did damage them, sometimes. You need enough dps to take them out before they kill you. You may not be able to heal a person constantly, and out survive the enemy just by healing and tanking.
You need good damage delivery, take out adds in certain fights first and tank the Elite or champion guy to kill last. You simply may not be able to tank everything continually, just keep them from the rest of the group and kill them fast.
Blue gear suited to your roll helps. High endurance is key for tanks, its also nice if you have defence, but I have trouble finding and crafting things with defense, just focus on Endurance armor, and a good shield generator. Actually taking Tanking talents helps a lot, learning what abilities are important to tanking helps, dont just blow your cooldowns at any moment, use them when you expect to take high damage, not as you pull but once they are attacking you you use it.