Why have my members gone onto a waiting list?:
When enabling RSVP's for an event, you can set a reply limit. Anyone who replies 'yes' after this limit is reached will go onto a waiting list, which can be viewed in the details of the event. If a member who said 'yes' changes to 'no', the first member on the waiting list will go into 'yes'.
If everyone is on a waiting list, it's likely the admin has inadvertently set an RSVP limit of zero. Check this via your team's website dashboard or app by 'editing' the event and updating the RSVP limit.