Try to setup a calendar system in your website in which people can make reservation for specific day and time that you are available. Make the reservation time available to the public, of course anonymously, so that if someone wants to pinch in, he can see if he can make a later time of the same date. He will then be "motivated" to pinch in at the days that have 1 or 2 people already, assuming that he is available on multiple occasions. Once say 3 or 4 people are filled for the time slot you want, you can contact them regarding payment, etc.
Example: I really feel like banging a blonde with green eyes this weekend. Then I check to see the calendar if you are available on say Saturday. If no one made prior reservation, I create one. If someone already did, I pinch in. Once 3 or 4 people fill up, you would let us know via email and we will work out the payment. If the slot does not fill up, you will still let us know to see if we can pinch in at later date.
Congratulations on your new business venture! Ok I need to stop checking TER every 2 mins into my classes.