Hi - I help run a social impact experience and we would also like a way to do this. We lose money/donations each time we host an experience with just one guest. We still want individuals to have the option to book our experience so maybe the following system could be a solution if airbnb would implement it:
For the experience page the host would set a minimum guest category.
When a guest books they will have a message explaining that alone they do not meet the minimum of people so they can either choose to have
a) a pending booking (only goes ahead if the minimum book before that date)
b) pay upfront for the minimum people (with thier money refunded should others decide to book the experience)
There could be a second reminder one or two days before the experience if the minimum have not booked, giving the guest the option to cancel a pending booking or pay the minimum price to confirm it.