Keep it social with some competitive spirit thrown in.
With a larger group of different paces, pick an out and back trail and pick a time (not distance) that everyone will turn around at, such as 25 minutes. All run at their own pace heading out for 25 minutes and all try to negative split back to the start, the faster group will run further and still get a good strong finish feel and finish about the same time as the slower group who did not run as far down the trail, but still get to finish strong.
This sort of workout allows the group to start and finish together, promoting the social aspect before and after. Allows faster runners to not feel like they have to slow down the group. It also adds a little competitive spirit, even for slower runners (can I get back before the fast guys). Finally, in a triathlon run it is good to not start too fast, start easy off the bike and build throughout the run and finish strong. Pushing for a negative split, even in a training run, forces you to run faster on the second half of the run.
Go get recovery drinks after.