I have been running into issues with screen when switching to another user:

$ su otheruser
> password
$ screen -x
> Cannot open your terminal '/dev/ttys002' - please check.

From this post I have discovered that it was due to a permissions issue.

$ ls -l /dev/ttys002
> crw--w----  1 user  tty   16,   2 Dec 30 23:34 /dev/ttys002

To fix I had to set other permissions.

$ su otheruser
$ sudo chmod go+wr /dev/ttys002

You still have to su to the owners account and therefore it is still secure. ie user can not just connect to a /dev/ttys002 currently in use by otheruser.

 #As user, otheruser currently using /dev/ttys002
 $ screen -x /dev/ttys002
> Must run suid root for multiuser support.

The issue has kept recurring and I have had to reset the permissions every time I wanted to connect. This could be Mac OS X specific but I edited the root crontab to fix.

$ sudo crontab -e
# Added the text below
# MIN   HOUR   MDAY  MON  DOW   COMMAND 
  0,15,30,45 * * * * chmod o+wr /dev/ttys000
  0,15,30,45 * * * * chmod o+wr /dev/ttys001
  0,15,30,45 * * * * chmod o+wr /dev/ttys002
  0,15,30,45 * * * * chmod o+wr /dev/ttys003
  0,15,30,45 * * * * chmod o+wr /dev/ttys004
  0,15,30,45 * * * * chmod o+wr /dev/ttys005