Assigning Moderators

Just provide a list of the usernames seperated by commas. Only users which have the status 'Moderator' are actually saved, so you have to promote them to this status by editing their profile first. Although there is no harm done letting them keep that status after purging their name from the list of forum moderators (at least the default permissions of the 'Moderator' group don't differ from the 'Member' group), it's certainly good practice to change the status back to 'Member' again if you don't want a user to be a moderator anymore.