It's always going to be difficult in that situation; I'd personally encourage the GM to tend towards more open spaces, but of course that won't always be reasonable.
I would reserve a spot up front for a defender, to maintain marks on as much of their front line as possible. The remaining spot should cycle between characters: attack, then withdraw, with the defender covering the retreat in the usual fashion. This won't give everyone an attack every round, but it's a start.
Readying actions can help: one of the melee characters in the back who can't reach the front on their turn can ready to charge when there's room. Again, this won't totally solve the problem.
Bigger changes, which may require some rethinking of feat choices and such: some of your melee characters might be better off with polearms. My girlfriend plays a few melee healers, and she tends to prefer polearms with them because she can fall back into the second rank and attack with protection. Barbarians do fine with polearms, as do melee leaders, as do striker-oriented fighters. Oh, and assassins.
If you've got rogues and rangers among your party, they may want to think about the at-wills which allow them to move as part of the attack. That'll make cycling through the front line easier.
Finally, look for powers that allow you to force the enemy line back or that allow you to get behind the enemy line. Barbarians with Pressing Strike can go right through that line. Lots of classes have at-wills that push the enemy; move the enemy on the right back till he's directly south of the enemy on the left, and suddenly you've got a lot of room. Eladrin can teleport, as can assassins.
Or in a pinch, shock your GM by jumping over their front line. The vertical distance you clear with a long jump is 25% of the horizontal distance, so if the enemy is six feet tall, you'd need to clear 24 feet or 5 squares in order to get past them. With a running start, that's only a 25 on your Athletics check. Easier if you're higher level, admittedly. You might also be able to get your GM to allow an Acrobatics check to parkour along the wall and past them. I'd allow it but I'm a Hong Kong action movie fan.