This isn't that complicated.

The airline tries to get more volunteers then they will need so that they won't have to try to find more at the last minute. Volunteers are placed on the standy list in an order dependant on the airline's specific policy. After the check in deadline the agents will release the seats of passengers who haven't checked in and will start at the top of the list and work down. As they reach the last few available seats they may have to pick some volunteers our of order in order to match the number of passengers traveling together to the number of remaining seats. If that's not possible then they'll ask a group of volunteers if they'll split up.