In reply to tony:
We did a couple of interesting group sessions last year. They were closer to interval pace than tempo, but could be adapted.
The first was a variation on Parlauf sessions. It involved marking two routes (one of about 500m, the other of about 800m - in our case two blocks on a quiet housing estate). People would be paired up in roughly equal pace, and one would go out at interval pace around the larger route, whilst the partner ran the shorter one. The idea was to get to the end of the loops at the same time, and then switch, so as to create a continuous relay, for 40mins or so of total running.
The other involved about a 400m circuit, and groups of three similarly paced people. One of the group would start off, and at the end of the lap a second person would join them. At the end of the next lap the first person would drop out, and the third person would start. This would carry on, again as a continuous relay of everyone running two laps before one lap off, until the time was up.