Any creative project that requires rehearsals can and will be a logistical nightmare, no matter if you are dealing with 2 or 20 people's schedules. Finding days where every person can show up for practice at the same time will be frustrating. It's important to come at it with an organized plan of attack.
Take a thorough look at the project in question. Is it a 5-minute sketch with a one-time performance or is it a 2-act play that will run for several months at a theater? Is it tech heavy or completely tech free? Is the staging simple or will it involve intensive choreography? These needs will determine how many rehearsals your project needs.
With your calendar, make a dream schedule of rehearsals for yourself based on nothing but the best case scenario for availability for your cast. This should reflect the most rehearsals you feel are necessary for getting the production or project on its feet in time for the performance. Keep this calendar separate.
Give your cast members or project collaborators blank copies of a calendar that reflect the time in which you need to rehearse. For example, if you are beginning the process September 1st and the show goes on October 30th, give each member copies of September and October calenders. You can do this by photocopying a wall calendar, printing a calendar off the Internet, or even sending a electronic calendar via email to be filled out with personal computers and emailed back.
Instruct your cast members or project collaborators to fill out these calenders with their availability. Tell them that this should include specific hours of availability (free after 6:00 p.m.) as well as prior commitments that can be broken if the need exists (8:00 p.m. dinner plans that can be broken). Encourage your cast members or project collaborators to be as specific as they can.
Give your cast members a deadline, such as 24 hours, to get the calenders back to you. Be firm about this deadline.
When you have collected all the calenders, pull out the dream schedule that you created earlier, as well as a new clean calendar that will eventually be the master schedule. Go through all the schedules one by one and compare them to your dream calendar. Circle the dates in the dream calendar that correspond perfectly with the majority of the cast, and then note them in the master schedule.This will give you a starting point to plan the rest of the schedule.
Go through all of the schedules and make note of similar free days among people who need to rehearse together. You can do this first weekly, for example, if they all have Sundays free in the afternoon. Then move to a date specific calendar, for example if they all have September 5th, 12th and 15th free. Set these rehearsal dates next in the master schedule.
From this point you will have a better foundation to plan the rest of the rehearsals schedule. Consult your dream schedule and compare it to the master schedule. Note which rehearsals are not assigned dates or times yet and work your way from the front of the calendar to the performance until most or all of the necessary rehearsals are set in stone on the master calendar.
Look over your master calendar one more time, adding such details as rehearsal space addresses and scenes to be rehearsed. Also, make sure that the names of the actors or collaborators are clearly listed on the days they are called for rehearsal.
Save or photocopy your master schedule and disperse it among the cast or collaborators. Make it clear that this schedule is the final schedule and no other changes are expected.
Accept that it is sometimes impossible to get everyone at one place at the same time. Do the best that you can.
If a cast member or collaborators schedule simply doesn't fit with any other schedule, you may have to rethink including them in the project.