Ok, this is going to be a general introduction to teaming and team missions especially. These tips will keep you alive, keep your team alive, and make missions more enjoyable in general. Ok, in no particular order, here we go:
You have a mini-map. Use it. Make a trip to the maps booth in any clan shop and buy four things: Map Reader Upgrade - People, Map Reader Upgrade - Machines, Map Reader Upgrade - Monsters and Map Reader Upgrade - Directional Arrow. You should buy these and upload them as soon as possible. There's no need to raise map navigation for this. Save your ip, make some map navigation implants ql70ish and put them in as soon as possible. Now, when you're inside a mission, you should ALWAYS have this window open. This lets you see where monsters are and, more importantly, where your teammates are. They show up as little circles.
The team window, like your mini-map, should ALWAYS be open when you're inside a mission. This displays your team members' names as well as health and nano. Keep an eye on this. It's the only way you'll know if someone is taking damage unless you can visibly see them fighting.
Let the person with the most hp tank. This is typically an enforcer or keeper. If you're NOT the tank, hold back a second or two before you start fighting. Wait for your massive special attack (Aimed Shot!) until the tank can get a firm grip on agg. You are much less likely to die if you'll be patient. It's also a lot easier on the doc to heal one person instead of trying to keep up with everyone getting hit randomly. Whoever is the tank should also be the first person into a room with mobs. Don't run in and start shooting and then complain that the tank can't keep agg. Finally, if you're the tank, you should ALWAYS die before the healer. If the healer is dying first, something is wrong.
Know who your healer is. Listen to them. If the healer has to afk, running in and attacking a bunch of mobs is a guaranteed way to die. If they say they need to regain nano for a moment, wait for them. Remember your mini-map and team window? Keep an eye on the healer's location and status with these. If the healer starts to take damage, do your best to pull agg off of them. If the healer gets low on nano, hit them with a nano stim. This is in your own best interest. If the doctor goes down, there's a pretty good chance most of the team is going with them.
I know, your pet is awesome. It can probably solo these mobs by itself, especially if you're higher than level 100. Good for you. Even so, do NOT put your pet on /hunt. This wreaks havoc. Pet pathing has always, and will always, be horrible in a mission. Your pet will fight mobs across the level and miss the one beating on your face. Hopefully, you've discovered this already. If not, you're time is coming.
If you're a metaphysicist, ask the healer who they want the healpet on. If you have a team member that will pull agg consistently (like a shade) these are good people to put a heal pet on. Keep your mezz pet for emergencies. If you're high enough, you should be using Transcendent Emnity Personification instead of Cacodemon (unless your pet is the tank). TEP does a lot more damage at the price of hp.
You're in a team. Share the wealth. The default setting is loot all. This means anyone in the team can loot anything. Show some courtesy here. If you know it's a good item, ask if anyone needs it before looting. Don't be a punk and loot that rare doctor nano that you know your team's doctor needs. If it's a NODROP item, check that you need and/or can use it BEFORE looting. This is especially true in dungeons. If it's a team of random people, ask the leader to turn on 'team loot alpha'. This eliminates loot issues for the most part, and you can work out special cases as needed.
When you go up or down a level, you appear on the lift. Move. It's that simple. When you stand on it, you're blocking everyone else from exiting in a hurry if that's required. This is especially important in the boss room.
When chaos ensues
In any team, there's a chance that chaos will ensue. Somehow you pulled four monsters, you're taking a beating. Your instinct is to run. Don't. If you run out of range of the doctor, you can't get healed. Running around means you've got four monsters whose agg just went haywire. You're likely to pull even more mobs. Instead, do the right thing and stay still (you can run in tight little circles if you're sure there's no other mobs to agg). If you die, deal with it. No one likes dying, but the entire team will be happier if you die like a man rather than taking them down with you. Generally, try to keep your cool and everything will go smoother.
Read my rules. Abide by them. Don't team Shay when he's been drinking. You'll do well.