Ran into this myself some time ago. The problem is due to permissions on the "device" screen is using. This is why you are getting the "permission denied" error. Can't quite remember which device right now, but I wrote a script to change perms on the device for users wishing to do very close to what you want to do. If I recall, I brought up screen while using another session to poll (cd <device_directory>; while true; do ls -ltr; done), the device list. This way, I could see which device was assigned a user, and know which device to change perms on. Hope this helps... if I remember, I can look for the script tomorrow and post it.