To add a new password, use luksAddKey:
sudo cryptsetup luksAddKey /dev/sd?? (You fill in for ??)
To delete a key
cryptsetup luksDelKey /dev/sd?? 0
where 0 is the very first key ever created or added.
seems to imply that you can luksAddKey more than one key. Perhaps you can give different keys yo different users??
To remove an existing password, use luksRemoveKey:
sudo cryptsetup luksRemoveKey /dev/sd??