We use EventSpot to collect registrations for training classes. We have a hard limit on the number of attendees. However, it is not uncommon for someone to cancel. I with there was a wait list feature that kicked in automatically when the event limit is reached. Currently we manage this process entirely offline, which requires individuals to send a separate email to the person coordinating that training class, and then either manually register them or ask them to register later. If the registration button could change to a "Join Waiting List" button, then it would allow us to collect registration information from the people on the waitlist, and simply change their status to registered when space becomes available.
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