To invite someone off the waitlist, you can send them an invoice that they will pay online. Once they pay, they will be enrolled in the session, even if the number of available spots is zero.
You can learn how to send an invoice here.
If there is no fee for the session or the amount due is $0, you can change the status from Waitlist to Enrolled by using the Action button on the registration. Choose Enter Payment and enter the amount of $0.