The 4th edition version reads in part as follows:
Attacking Through an Occupied Hex
You can attack “through” someone else in melee if you are using a weapon with a reach of two yards or more. You may attack through a friend at no penalty (this is a basic part of your training with any long weapon). (Campaigns, p. 388)
Is that really true, though? When you learn staff, spear, two-handed sword, longsword, greataxe, maul, glaive, etc. you learn to swing over and around your friends?
Do your friends learn how to fight in a way that leaves you totally unimpeded to attack through them? And they aren't thrown off in any way, shape, or form by you attacking past them? All on a one second time scale?
I think not. I think earlier versions of GURPS assume some of what was later added under the Teamwork perk. I think a "harsh realism" rule is appropriate here.
Harsh Realism for Long Weapons
GURPS allows long weapons to attack through friendly hexes at no penalty; this rule applies a -4 penalty per occupied hex no matter the relationship to the attacking fighter. This penalty is eliminated if both the attacker and the occupant have the Teamwork perk.
On a critial miss, any "dropped weapon" may hit the occupant of the hex you are attacking through - 1-4 on 1d. This is distracting - -2 to all rolls for that turn (and may be harmful if the weapon is a force sword or similar weapon.) Critical misses that result in hitting a friend will hit the occupied hex's occupant . . . if it's an ally! Otherwise, determine who is hit normally.
Seems a little harsh, but not too harsh. I suppose I can harsh it up . . . but this is a good start. It seems better than you need to have learned to fight with someone before attacking over them without penalizing either of you.