I will sometimes tell the group to "remember where they came from" and how it can be prety discouraging to come to a "beginner level ride" and not be able to keep up. I try to activate the "shepherd" in at least a few of 'em. When I go to a beginner level ride, I ride my big fat bike and go slow even if I'm not the leader. I was always the last one in sports... and in my very first bike club tehre was a whole group of guys who woudl go slow for you so I'm giving it back, and I tell people that, too.
It's an ongoing thing, though. I also tell the new folks that one of the problems is that you get strong pretty fast and you don't realize it! So if they'll keep coming back, they'll be with those people in no time...