Well since this is just a group of friends, it's really up to each member to come up for a solution for their own problems (I'd be happy to help. Can't speak for anyone else). Speaking of which, most of the group doesn't play SWAT all that well. So I'm writing a mini-guide here for the SWAT players. I think everyone interested in playing should read it, some to get an idea of what's going through my head while leading, others to criticize.
Simple Guide to SWAT for GitP
The game is heavily based on teamwork, so it's important for everyone to know what to do. If you're not confident in your abilities, don't be afraid to ask for a normal difficulty game (the lowest). Also I find that soloing (playing through the game alone) increases your own ability the most, so consider trying it.
The zombies have unlimited resources, their units continually spawn throughout the game with various numbers, hitpoint totals, and damage based only on the number of players, their experience levels, and the difficulty level of the game. Don't listen to anyone (I can think of one in particular who has said this) who tells you that radiation fragment count influences the number of enemies that spawn; those fragments only increase enemy health and mana regeneration which gives them access to new abilities (plus there's the slow draining of allies' health). I've verified this with information from the game creator's website and an experienced player. While the rad. count increases exponentially, there's also a bonus for their growth at the start of the game which makes the rad. objective difficult to finish early, so there are some situations where you'd best ignore them.
So, because zombies have unlimited resources, the group must use hit-and-run tactics. Be extra careful to avoid wasting the team's resources needlessly (batteries, energy, skills, dying), always run from enemies instead of taking a few hits early on. Nothing should have a chance of hitting you until flying gargoyle enemies show up. Even tanking classes should avoid getting hit before their nanites are maxed, they'll need the energy later. Every hero has a base 6 energy regeneration per second though, so it's worthwhile to invest some in an ability like item teleport or battery charging.
But the best advantage that SWAT players have is the ability to run. You can just about always move faster than the common zombie, only in extreme difficulties do heavy armor characters get in trouble for being slower than the mob. There's one critical thing for the player to remember though: by entering a room for any extended amount of time, you're temporarily giving up your ability to run.
Zombies spawned in rooms (anywhere on the map) or the horde following your every step can and will catch up to you, making your exit more difficult than usual. I have lost count of the number of games I've lost due to staying in the lab for a couple extra seconds or trying to open up some crates.
It makes me wince when half of one of our groups goes into a room in a mad rush for some drugs or something. Before entering the lab, think about what items you're going to buy and be prepared to click on them quickly. Before destroying crates in a room, think about what you hope to find. If the group can't open crates quickly, only open one and ignore the brown ones if you see implants. Only open silver crates if you can greatly benefit from a particular item (mfg, rapid reload, combat vest) or if you're somewhat sure there's an ATME. If the group can open crates quickly, send only the opener and perhaps the fastest second character in. Drop items outside to open inventory space, there's a good chance someone else will be willing to take it. You can trade items among the group faster and more safely when you get out.
So it's very important that you minimize the amount of time spent in rooms.
While zombies do have unlimited resources, they don't have unlimited control of the map. Zombies mainly spawn in the graveyard (north part of the map), so it usually takes a while for them to come attack you. The enemies spawned in rooms only provide token resistance for a good group, but they must be put down quickly to regain the ability to run from the mob when needed. So, unless your toon is in danger or far from enemies (performing objectives), I recommend always attacking. It relives pressure to allies and gives breathing space, plus valuable experience.
Experience is the only player resource easily gained, making it very important to your success. Maximize the benefit of your experience by becoming familiar with your character's skills.
There are really only two ways to begin a game of SWAT: either camp near your starting bunker for five minutes or so to gain experience or rush into filling one of the reactors ('tor). I much prefer the former because fast-moving enemies spawn within the first 4-5 or so minutes of the game and demand response from the players.
The leader should camp periodically to get maximum amount experience for the group for the least amount of time spent camping. Focus on killing zombies quickly at this time, if you have a splash weapon, fire it into the middle of a group of zombies to maximize damage done but also be aware of your range so you don't go running into the mob. Camping doesn't only give experience, it's required to prevent the special Tyrant (a kind of enemy assault droid) enemy from spawning when the number of zombies gets too huge. Sometimes this is unavoidable as well.
Also, the civilian objective has been changed recently. Sometimes a converted zombie will only be "partially inoculated" and require a "redote". Just use the antidote on it again and it should work, this doesn't cost an antidote charge thankfully. In higher difficulties, sometimes civilians will need double redoting from more than one player, be aware! You can tell when your zombies need redotes by the red "debuff" portrait on them, or by seeing faerie fire sparkles around them on the screen.
Edit: I found this useful.
As I've said before, there is no benefit to a person re-doting the same zombie more than once. The process should always be 1) antidote a hostile zombie and 2) everyone re-dote it once. That's it. That's all you can do to try to convert a zombie into a civilian. Not really that complicated of a mechanic.
In NM/EXT, it will be impossible to convert some of the zombies.
Finally, reactors should be approached carefully in groups. There's a small chance for them to explode when they beam energy away from you, and also a chance for the repair droid to become hostile and attack you. To prevent 'tor explosion, avoid transferring more than 1500 energy into it while in a large player count games. If the repair droid starts attacking you, run out of the 'tor and tell the next person to go in. Hopefully the rest of the group can destroy it, giving you a chance to go back in. Never have more than two players in a 'tor room.
One more thing for soloers: you can use the Temporal Avatar item to attack while moving at the same time. Just put out your avatars, then use an ability that the avatar cannot use, like pickup or drop an item, or one of your skills or innate abilities, so that your character moves within range of wherever you want to go. With some practice, you can prevent yourself from actually performing that action with a stop, move, hold position, another skill, etc. All the while, the avatars will be attacking anything visible within their range (so they won't attack enemies you can't see, maybe use a camera, c4, robodog, or LAD to remedy this). I can't dream of soloing without it.
Now accepting criticism. Turned out kinda long, meh.