You may run into a problem where even after adding transmission to the users group, that you cannot delete or modify files downloaded from Transmission. If this happens, you can add the user(s) you want to modify the downloaded files by running the follow command as root replacing "admin" with the user you want to add:

Note: You need to stop the transmission-daemon service and watchdog before running the above command, and a restart of the server may be necessary, just remember to restart the service and watchdog first.