Setting expectations in advance is helpful...if you're planning on a "keep up with traffic, and pretend we're in Idaho with regards to stop signs and traffic lights" ride, just let people know in advance.
Set rules for regrouping; like, for example regroup at the top of hills or at any turns.
Cue sheets are good...people can always refer to them later if/when they want to ride the route themselves, even if they're just planning to follow the leader.
Determine break/rest points in advance, and put 'em on the cue sheet, or at least let people know where the planned stop(s) are.
If you have people of widely divergent abilities, such that regrouping at regular points is onerous, think about splitting it into two rides. Having at least one other person along who knows the route and can take over if needed is really helpful.
Maybe get someone else who knows your route to ride sweep (follow behind everyone), so they can make sure that everyone's okay, and so that all your riders know that there's someone with a clue behind them.
Check out some of the city rides, like the Tazza d'Oro Team Caffeine/Decaf rides, or the Major Taylor club rides. That'll give you a good feel for group ride dynamics, plus you can ask the group leaders for advice first-hand.