Workshop - Add to Waitlist Option

I would like students to be able to add themselves to a waitlist in the workshop to pick up a spot if a booked student changes their mind. Not having the ability for them to put themselves down for waitlist could mean that we have workshops that are under booked regularly and having a list of students who are available and want to go would solve this problem. 

If the workshop booking allowed us to have Minimum and maximum numbers, but then also add a third option for how many students could be on the waitlist - this would be very helpful!