When grouping it is often not as critical to be multi-professional as it is when soloing. Yes, if you are a Medic it still helps if you are an Entertainer and Scout as well, just for the sake of being able to dance away mind wounds and heal wounds in a camp that you can create, but it is not entirely necessary - if you group with the right people.
If you are playing as a melee-oriented profession in a group then it will be your job to make sure that you take all the damage that you can, in order to ensure that the rest of your group takes as little as possible. And while it sounds like this is painful, it is even more so in practice.
Additionally, while you are up there taking that very same beating, you will want to, if you are being attacked by a group, focus on things that will affect multiple enemies. This is known as crowd control. While the pikeman is best suited for this, the other melee-oriented professions are capable as well, as they level up. When you are not attempting to control the crowds, you will want to fight as if you were soloing, because when you're taking point it can sometimes feel very lonely if your medic isn't able to keep a constant stream of healing. Remember that blinding and stunning each enemy in turn will help you live longer. Don't focus on damaging all of them, the rest of your group is doing that while they shoot them; just blind and stun everyone and you will survive.
In a group situation, ranged fighters generally have the job of dishing out damage. The Brawlers (with all the close-range defense skills) will be up front taking the hits and trying to stay alive long enough for the ranged folk to do enough damage to drop the targets.
For the most part, you will be fighting the same way you would if solo, save that you won't be doing too much kiting. Ideally, enemies should never get close enough to you to require kiting. Stay back, not at maximum range (because of the huge penalties), but far enough that the enemies won't pay too much attention to you and turn away from the brawlers. Shoot a lot, mixing in a special attack now and then--especially attacks that stun or blind enemies--but don't waste all your HAM points. If things suddenly go bad, say, if the brawler goes down, having enough points left over to pile on a couple of high-damage special attacks and end the fight quickly can be enough to save the day.
Finally, pay attention to the group leader. They will determine what targets to attack most of the time. Make sure that you are attacking that target. Failure to do so can get entire groups wiped out as you waste your attacks on weaker creatures or bring otherwise non-fighting creatures into the battle.
It is when grouping that the support professions really shine. Those who are entertainers can relieve mind wounds in camps, those who are scouts can setup the ultra-necessary camps between battles, and those who are medics are of the utmost need and are often truly sought-after.
It is because of this that playing a support profession is a very serious thing, as often the entire fate of the group will hang on your shoulders. If a medic doesn't pay attention, or doesn't heal at the critical time, everyone in a group can sometimes end up dying very fast. As a result, the best advice that anyone can give you is to stay focused and watch your teammates. The moment you start to see their stats drop too far, heal them, and bring their stats back up.
Or, if you are one of the other support professions, look for opportunities for you to show off your specialty; your group will thank you for it.