If you choose the “Maximum participants” option for an event, the first members to accept the invitations (to the limit the administrator has decided) will be able to attend. Members will be able to put themselves on a waiting list when the number of spaces has been filled.
If someone attending withdraws from the event, the first person on the waiting list will receive a push notification regarding the free space, and will be able to attend. If he or she declines, the next person on the waiting list will receive the offer and so on.
The administrator can not overwrite the waiting list by moving someone up the list, it works by the "first in, best dressed" principal. Admin can although set members on the waiting list, decline and remove them by going into the attendance list.
To disable the max number/waiting list, go to the event > edit > press "remove" next to the max attendees number.
If there is a registration fee for the event, the member will not be asked to pay until they have accepted their place.