I'm compiling a guide on groupfighting for people in the Reddit Brigade, and I'd love your input. Here's what I have so far.
In the beginning (of the fight, as well as when you're a groupfighting noob), block. Nothing sucks more than dying to the first stab you come into because you misjudged the situation. When you get better, you'll be able to better judge when to release an attack and kill someone who's going to try to stab you. But it never hurts to block if you're only being pressured by one person.
Move around a lot. Because enemies are all around you, you need to make yourself a harder-to-hit target. The best way to do this is to not stay still for any moment. I can't tell you the number of times that I've died in a groupfight simply because I got stopped (whether on an obstacle or by a glance off an enemy or a blocked attack). And know that, once you get good at it, you'll do a lot more moving than blocking (which doesn't mean you should never block of course).
Be conscious of your surroundings. Because enemies are all around you, you need to know where you can go, who you can attack, or what direction you need to block. For this reason, if I'm not immediately pressed by enemies, I'll take a very brief moment to do a 360 and take stock of the situation. If you can avoid being caught by surprise, you have a MUCH better chance of surviving. It'll also help you to catch enemies by surprise (see below)
Attack unsuspecting enemies whenever possible. When you're outnumbered, with enemies closely surrounding you, you can thin out the herd by suddenly turning and stabbing an enemy following you. A good portion of the player-base will fall for this trick, as they'll be following you assuming that you're focusing on someone else. When you catch one person off-guard, you often will have the opportunity to chain more attacks, because the enemy will think you're vulnerable. In that situation, keep moving and stabbing. (I got 5 melee kills in the space of about 7 seconds at an LB yesterday just from chaining together kills beginning when I caught someone off-guard. "It really really works!")
That said, don't be that unsuspecting enemy. If you're in close proximity to an outnumbered enemy, be ready to block whenever at any point. You don't necessarily need to hold a block, but pay attention to what the enemy's doing. You'll have a window when they turn with an attack at which you'll have PLENTY of time to get a block up. But you have to pay attention.
Use your allies as distractions for a target. A great way to get kills in groupfighting is to wait for an enemy to get engaged with an ally, and just swoop in and stab him when he moves into range of you. Of course, this doesn't mean you shouldn't be prepared to block (as good groupfighters will take the opportunity to get a stab on you if you're just waiting for them to come into range and don't have your wits about you). But it often happens that an ally and an enemy will be fighting in a counter-clockwise motion, and you can just wait for them to come into range while they're busy blocking. Of course, you can also be the distraction while your ally gets the kill. In a line battle, all that matters is that your line comes out on top. The overarching idea is to gang up on enemies as much as possible, because if you can swarm an enemy (and not TK), he will be hard-pressed to survive.
Don't focus too heavily on going after a single target when there are plenty to choose from. If you get tunnel vision and go after just one guy, you're BEGGING to get stabbed, because you start to lose your sense of awareness. If you keep shifting targets, that means you keep moving, and that makes you hard to hit.
Be unpredictable. This is my most general and most important piece of advice. A player moving unpredictably and going after (seemingly) random targets is much more likely to survive than someone who always does the same thing. Don't do the same attack every time, the same spin, the same movement.
To steal Hekko's summarising format, let's sum this up:
Do block (at least at the beginning)
Do move around a lot
Do pay attention to what's going on around you
Do attempt to catch enemies by surprise
Do not get caught by surprise
Do use your allies to help you out
Do not target a single person when there are many
Do be unpredictable