1) Get permission first. Be sure that the governing body of the cemetery (church, town board, cemetery association) you plan on working in "signs off" on your clean up plan. You need permission to clean a gravestone first, whether you're cleaning 1 gravestone or 100.
2) Have the volunteers sign a "hold harmless" waiver. There are plenty of opportunities for folks to get injured when working in cemeteries, from tripping hazards to bee stings. Be sure to have your team sign an agreement to not hold your non-profit responsible for any injuries incurred during the clean up day.
3) Pre-select the gravestones to be cleaned. Not all gravestones should be cleaned. Some are too fragile and cleaning might damage them. Or, some are not secure enough to clean and may even topple over. Have a trained member of your team go out ahead of time and flag the gravestones that are safe to clean before you let loose your eager crew.
4) Train your crew. So, you have an energetic crew of young workers willing to get their hands dirty for the sake of cemetery preservation. You can't just let them loose with brushes, water, and D/2 in a cemetery. They need to be properly trained first...and then continually supervised throughout the day.
Remember that it all comes down to DO NO HARM--not to self, not to gravestones.
Oh, and "thank you" to everyone who cares enough about our cemeteries to organize these clean up efforts.