So you're being your friends in a fight, and you want to move up. However, they're filling all of the open hexes between you and where you want to go.
As written, the GURPS movement rules allow you to move freely through a friendly-occupied hex.
Let's look at an ASCII-art example, since I'm way too lazy to set up minis and try to take good pictures.
That means you (u) can go from behind your friends a, b, c, d, e, and f here:
to in front of them here:
. . . as if the hallway was empty. You don't disrupt any of the actions or concentration of your friends. You don't disrupt the defense or offense of any of them, either. In fact, (c) can retreat from an attack immediately if that occurs - say, if you roll a critical failure and hit (c) by mistake.
I've always found that unlikely at best.
I prefer to treat it as Evading (per Basic Set: Campaigns, p. 368).
Your ally doesn't have to oppose your attempt. However, if you come from behind your friend, you do have to evade normally - you can't automatically allow someone to pass you.
You roll DX; apply the modifiers normally.
For example - A is behind B. B wants to get in front. B must roll DX, -5 for foe standing up, +5 because you're standing. Failure means A is in the way, and you cannot pass. Critical failure means you bumped into your friend, putting one or both of you off-balance (1-3 you, 4-6 them). You're considered to be in the same hex as your friend, and behind him or her - your ally can't Retreat, or Step back.
Attempting "squeeze between" your friends is possible - that's what this represents. Your allies can move to let you aside - we treat that as a step, although you don't leave your hex. You just flatten against a wall, turn sideways, etc. This is not conducive to good defense - you defend at -2, and attack at -4.
Let me through! - Clever players will quickly say they're using voice coordination - "I say, 'Passing on the right'" "I'll let him by when I hear that." You can allow this - I generally do, but I require a Hearing roll and I require a turn of prep. With overlapping 1-second turns, you've got a fraction of a second to adjust to this request while executing everything else on your turn. Much like handing an item from one person to another, it takes time and coordination and isn't something you can execute in a fraction of a second.
This applies to NPCs, as well. Unintelligent ones will often fail to take advantage by attempting to rotating in fresh fighters to the front ranks, or pull out the wounded. Berserkers won't even try. Tactically sound folks will use it as best they can.
But often, the PCs have been able to keep a group of foes on their heels because wounded, stunned, or otherwise weakened front-rank foes aren't able to have friends rotate in for them. It's possible to trap someone against their own friends.
Without it, technically, you can fight in a one-hex wide corridor and have both sides constantly rotate in fresh fighters if they're not very closely pressed. I'm not saying anyone would allow a guy with Move 6 to run through 5 hexes of friends in a one-yard wide corridor, but it's at least possible (and automatic!) by the letter of the rules.
How is it in Actual Play?
This has worked well. I've had zero pushback on the rule. Occasionally questions about ability to move or not move in some fashion, yes. Actual disagreement with the play of the rule? None. Getting a 50/50 chance of passing your friend from behind without they even having to adjust to your movement seems reasonably and generous. So is the prohibition against getting through close combat. You could get through in a non-struggling situation given time, of course but when it's a struggle it's harder. You can push through a packed concert, but getting through an armed mosh pit without slowing down feels better with a lot of DX rolls, versus friend and foe alike.
Overall, I recommend this approach.