Someone cancels the 6.30 pilates at 4.15. There are nine people on the waitlist. The receptionist is teaching until 5. By the time anyone looks, it is 5.20 and the first three on the list have made other plans.
The booking system already knows about the cancellation the second it happens. Mindbody, ClassPass and most of the others fire a webhook or at least send an email. The system catches that and messages the first person on the waitlist by WhatsApp. 'A spot has opened in 6.30 Reformer tonight. Reply YES in the next 15 minutes to take it.' If they reply, the booking is made in their name and the next person is never bothered. If 15 minutes pass with nothing, the offer moves to number two.
Two details matter. The order is the real waitlist order, so the person who joined first gets first refusal and nobody can argue about it. And the window shrinks as the class gets closer: 15 minutes at 4pm, 5 minutes at 6pm, and after 6.15 the system stops and leaves the spot open. Nobody wants a text about a class that starts in ten minutes.
Every offer and every reply is logged in a sheet, so when a member says she was never offered a space, the owner can see that she was, at 4.16, and did not reply.
This is the kind of thing we build. It runs in your own booking system and your own WhatsApp number. Nine people on the list and one spot: how many of them would you have texted by hand?