I can try myself first.
EDIT: I solved it!
Hightlight:
Have the first lemming on the top dig on the right, but make sure it's over the right wall with the space underneath. The lemming on the top behind it must trap the others by digging and building. Meanwhile, on the bottom, the first lemming that comes out builds 3 steps, then blocks. Wait until the top lemming finishes digging so the others could climb it. Only one must climb the newly dug passage, however. After assigning this to that one lemming on the bottom and he climbs to the top he must dig a few pixels, mine a little bit, build to turn, and build across the gap. Then he builds two bridges to the exit. Then, to escape, the top lemmings should become athletes. The bottom's a bit trickier. You have to make the walking lemmings climbers, except for one. That one must bash to free the blocker. The freed blocker must build a bridge, then mine back. Same for the one that mined. Then make them climbers.
EDIT2: I now made the seven-builder-solution in my fourth set!