(No, this is not a guild.)
Almost everyone has done combat. Most people have been in a group. Some people have even done combat in a group. This isn't easy, as you can imagine. Sometimes it makes it harder. But it all comes down to how you work. If you branch of in many directions, that is usually when you die. But if you plan, it will work. Here is my ideal battle formation:
IN THE FRONT: Melee with AoEs-Because monsters may loop back, an AoE is vital, especially since there may be enemies on both sides.
IN THE MIDDLE: Healers-they can reach others better.
BRINGING UP THE REAR: Melee Front-Hitters and Ranged-Range is the only suitable thing to fill here, just like front-hitters who will get useless too far ahead.
I can't think of more at the moment. If you want some specific instructions for a particular fight, post here!