I am a mountain bike docent for The Nature Conservancy and lead rides each week, generally for new or slower paced riders.
Just a suggestion, for the first ride, make sure you have a partner (or two or three), someone who can sweep the group. Decide what you and the the other riders want out of the ride but you establish the parameters. I generally make it very clear "this ride is going to be slow paced, non-drop, that means we may have frequent stops. If being patience is something you can't do, then it's OK to bail, our feelings won't be hurt."
I would give a brief overview as to what the ride is going to be like. How much climbing, trail hazards, wildlife issues, distance of ride, etc. Then if any of the riders feels uncomfortable and wants to bail before the ride then have a choice. Make sure all the riders also know general riding rules, like calling out "stopping" or watching a line. Also check out everyone's equipment and make sure bikes are in order and the riders have water etc.
Do the first ride slow, at an even pace to see how all the riders do. I usually call out things like "big hill coming" or "steep rocky descent" so the riders a few folks back know what's coming.
Good luck. Group rides are fun but there is a compromise involved. In order for it to be a group ride, the fast folks need to slow down and be patient with the new or slow folks. As time goes on, everyone gets better.