The best thing you can do as a team is be coordinated. Communication and organisation is key to success. You'll have to be strict, but it makes such a big difference, you won't regret it.
First of all, weapons. The tesla cannon is great for clumsy team members, as it's impossible to waste ammo with it, and it won't hurt team mates. In fact, I'd generally recommend it for anyone doing almost anything apart from crowd control or burning biomass.
The most harmful thing in the game is any enemy someone doesn't notice. If they sneak up to your team via a blind spot, then they can start attacking, and there's a good chance team mates reacting will hit the alien's victim too. So this is exactly what we want to avoid. And we can do that by simply having the team stay in formation.
The easiest way to organise the team is to establish formations and stick to them. Everyone knows where they should be relative to their team mates, and from there can pretty much look after their own firing line.
1 is the pointman, and should carry a heavy weapon (minigun, autogun, etc.) to dive head first into whatever trouble you're approaching. 2 and 3 should have medium weapons (assault rifle, shotgun, PDWs, tesla cannon) to cover the sides, and 4 cover the rear with a medium weapon too.
In the event that one person can't handle what comes their way, then the two closest team mates can turn and give some support, adjusting their position if necessary to keep their team mates out of their firing line. However the person on the opposite side to this should NOT turn around and shoot, or else risk friendly fire and enemies sneaking up on them.
Bear in mind you can and should turn the entire team, so you always have your point man facing in the direction you're moving.
For longer maps (such as residential area), a dedicated medic (healing beacons + healing gun) might be required, who will not be able to shoot any enemies. Therefore it would be a waste of time to put the medic on one of those four corners. Instead, there is a triangular formation you should adopt instead.
A flat side of this triangle should take point, focusing more fire on where you're headed, and one focused on the pile of bodies you just killed. If you're fleeing from a large amount of enemies, then this formation can be easily turned around.
With the medic placed firmly in the middle, he can heal all his team mates without trouble too. And of course, redirect fire as necessary.
This is a great formation if one person dies, or you only started with a three man team aswell, if you disregard the medic in the middle. If in a four man team with a dedicated medic and the medic dies however, chances are you're fucked. You only ever have a dedicated medic because you need one.
In tight spaces, these formations can be difficult to maintain. So the best thing you can do is have someone covering the front, someone covering the rear, and just move in single file.
If you happen to run into any trouble, just stop, and adopt this 'staggered' formation. This'll enable two people to fire in each direction. This formation is more difficult to hold however, and more prone to friendly fire. Don't move unless you absolutely have to, and if you do, hold shift to move slower.
There will be occassions when you need to break the formation though, such as when you meet a shield bug. Just surround them, and attack from multiple directions. If possible, freeze/stun it. Once it's dead, get straight back into formation. It's the same sort of thing if there's more than one, you just have to make sure the team's in good communcation, and always knows which target to focus on.
Sad panda's shield legs don't work when sad panda's flanked.
If a spitter spits at you, don't be afraid to break the formation by rolling. It takes less than a second to get back into formation. You'll have plenty of time to kill the spitter, and you'll save yourself some health.
This is usually a really shit idea. As how much resistance you meet doesn't vary based on how many of you there are. It's a high risk strategy only worth using in speed runs, so I'll go ahead and quickly go over it. I'm also going to assume you want to split up because you are doing a speed run.
Plan ahead when you want to split up. Because any confusion when you get to a junction and just shout "split up, guys!", you could easily have 3 people go one way and 1 go the other. This can lead to one or both of two things; A) The guy on his own will die, or B) You'll lose time correcting the mistake.
Establish who's going where when you split the team while you're in the lobby. If you do this and your team doesn't consist of fucking idiots, you'll be fine.
Parasites hurt. A lot. If someone is infested by one or more parasites, they're dead unless a medic can get to them. Flame throwers are a good defense against them, and medics are a must have in case the flame throws don't work. If you have electric armor, a shock from that will do a lot of damage to the parasite, and could easily save your life.
Depending on the team's exposure to parasites, it might be worth bringing a dedicated medic. This is certainly the case on the Residential Area level. Protect the medic, ensure there's somebody armed between him and every alien in the area. Although he can save your life if you end up infested, he can't do it many times. Infestations are a huge drain on healing supplies.
A particularely brutal last resort if there's no chance you can heal is to run somewhere far away from your team where they won't be going before they kill you. If a parasite kills someone, that parasite will duplicate several times. And a bunch more parasites is the last thing you want near your team.
Dealing with parasites in tight spaces isn't much more difficult, however. Put the medic behind your point man, and he can provide healing to the one most likely to become infested.
There's also good formations to take when backed up against a wall or in a corner, which don't take any explaining. The same principals apply; make sure you can shoot in every possible direction, and make sure your medic can heal anyone if you've got one.
Good luck. You're gonna need it if you're playing on insane. If you're on easy or normal, then you really don't need to worry about formations. You'd also be a great big wuss. Grow some guts you sissy.
Also Blogspot needs more 'read more' buttons.