Given that Infected do not fight defensively in any sense of the word, as soon as you add at least one other person you can count on and coordinate with, your odds of surviving a fight skyrocket. Two-on-one tactics are simple, whoever has aggro goes 100% into "noisy parry mode" (verbal taunting sometimes holds aggro even if your partner is stabbing them in the back), and the other cuts them to shreds from behind. If aggro switches, you switch too (it's very important for whoever does *not* have aggro to be ready to receive it and start parrying at any second - if they get caught by surprise and go down, two-on-one has become one-on-one escort with an *injured* sheep - fail.
If you've got the speed to stay ahead of the Infected, and your companion(s) have ranged weapons (*that they are skilled with*), a lower-risk tactic than head-to-head melee is for you to run the Infected in wide-ish circles around your team mate(s) while they range it/them behind you. They should stay quiet - the firing sounds should *not* trigger anyone, only actually hitting them, which if they were skilled means it doesn't matter since they're disabled. Still, only do this if the shooter is A) comfortable they're not going to miss and hit you and B) they are able and ready to fall back to two-on-one if the aggro management goes south.
This is the hard-core classical/medieval notion - you stand shoulder-to-shoulder. Parrying is easier because you can help guard the people to your sides and vice versa, and repostes are easier because you have up to three people looking for openings rather than one. Combined with shields and discipline, this brutally fucks any uncoordinated assault that doesn't involve guns/bombs/weird terrain.
If a pair are seriously outnumbered such that maneuvering becomes risky, standing back to back gives them at least a chance at making it out. You just have to remember to parry *everything* - no dodging, because anything you dodge may hit your partner behind you. But if you fight conservatively and riposte carefully, two guys back to back can come out on top over half a dozen Infected. I've seen it, and it's bad-ass. I've also seen one of the pair go down and then they're both instantly ass-raped, because they're pre-surrounded. This can work, but it's really last resort I'd say - stay mobile as long as you can.
You guys have the properly-working brains, so *use them*. If you are able to *anticipate* a combat, take a minute to review the situation and make a specific plan - it may not last long, but it's a good advantage in the early seconds of a fight. If your group has regular down-time in a secure-ish location (if not, you have guards posted, right?), then use some of it to think about/practice various scenarios and maneuvers. It can turn an otherwise chaotic battle into a series of mini-plans, enacted on the fly in response to changing situations. "Phil, Back-to-back!" "On three, circle the blue-shirt guy!" "Adds incoming West, 20 yards!" Even if these tactics are only marginally better than a FFA, every bit counts - plus, this sort of thing helps add confidence in what just a couple years ago might have been a shit-yourself weeping kind of situation. Just being able to keep your head in combat is a huge advantage.