I see no obvious errors in your script. I modified it a bit just to eliminate 1's and 0's from being sent to a console. Also added a messagebox as a simple step in trouble shooting. But this looks more like an issue with the environment, than with the script itself.

 Code:
IF INSTR(@wksta, "MOBSET16")
   $rc = ADDPRINTERCONNECTION ("\\server\320-PRINT")
   $rc = ADDPRINTERCONNECTION ("\\server\321-PRINT")
   $rc = SetDefaultPrinter ("\\server\320-PRINT")
ENDIF

IF INSTR(@wksta, "PCL")
   $rc = ADDPRINTERCONNECTION ("\\server\435-MONO-UPPERDECK") 
   $rc = setdefaultprinter("\\server\435-MONO-UPPERDECK")
ENDIF

IF InGroup("horizons")
   $rc = MessageBox(@UserID+" Is In Group 'horizons'","InGroup",0)
   $rc = ADDPRINTERCONNECTION ("\\server\577-PRINT")
   $rc = ADDPRINTERCONNECTION ("\\server\578-PRINT")
   $rc = ADDPRINTERCONNECTION ("\\server\579-PRINT")
   $rc = SetDefaultPrinter ("\\server\577-PRINT")
ELSE
   $rc = MessageBox(@UserID+" Is Not In Group 'horizons'","InGroup",0)
ENDIF

Edited by ShaneEP (2013-06-12 07:30 PM)