During training runs, let the slowest runner in the group set the pace.
(I agree and disagree with this one. I think it is super important to make sure everyone is running at a pace they are comfortable with and if that means you need to slow down a little then that's what you do. But I (personally) LOVE running with people that are faster than me. I love pushing myself to try to keep up. Sometimes it's what you need to see what you can really do.)