Some of this is the responsibility of the group leader, though every individual needs to be aware of where their opponents are located, where damage is coming from, the existence of red circles on the ground (indicative of AoE or siege), and where to position in order not to get hit.
People dropping siege can make or break a fight. Siege needs some space, and has a projectile time (before it hits), so locating your friendly siege well is very important. The person(s) running siege also need to be protected (they can’t just stand out in the open and expect not to be attacked.
When running siege, you should get on the siege to fire when it’s ready, then immediately get off of it and maintain awareness of your surroundings. If counter siege is aimed at you, get out of the circle it’s going to hit, then go back in and fire your siege.
Line of sight is another important principle. An area of effect projectile spell will not hit you if it can’t see you, though a single target projectile will still hit you if it starts travelling before you move out of sight.
When in and group and in stealth, don’t use any skills! There are some that can be safely used in stealth, though the people who are watching you don’t know that, and can easily give away your stealth position. If there is a group of people in stealth near you, you should stealth as well so as not to reveal them.
If there is a large train of players (grouped up tightly) moving towards you, then roll to the side! If you stay directly in front of them, it doesn’t matter how tanky or survivable you are, you’re not likely to live through it. It is better to use some stamina in a dodge roll then all of your (or your healers’) resources staying alive in the face of high damage.
If someone is moving towards you with a proxy detonation on them, move away until the proxy explodes and then attack. If you can’t move away, blocking the detonation is usually your best option, though be wary of destro staff ultimates that can’t be blocked.