You had me at a full moon ride and ice cream
I agree with N.Y. on the initial email. Having info about safety and it being the major concern in that email would help too. That should be a ‘copy and paste’ case in any size group ride email though. Assuming the ride is on Friday night’s full moon you could still get an email reply to participants out about safety and yes, the possible large number of riders and a staggered start if necessary. For 100+ riders having three groups, fast, medium and leisurely could help. Managing a crowd of cyclists eager to ride can be a daunting challenge though. I’d look at it as a learning experience if things don’t work smoothly this time knowing it will get easier in the future with consistency.