Sure you can, but why the conditional check on Addprinterconnection? There is nothing the script does either if or if not, so you can also omit that.

Back to your problem, I'd go ahead and place some check marks in there to see what happens :

Code:
select

    Case $Gateway="10.113.0.1"
        "We're in CASE Gateway=10.113.0.1" ?
        If InGroup("Pri-NL-Administratie")
            "And yes, we're ingroup Pri-NL-Administratie" ?
            $nul = AddPrinterConnection ("\\amssrv050\HP4300 IFS Administratie PCL6")
        EndIf

    Case $Gateway="10.113.0.1"
        "We're in CASE Gateway=10.113.0.1, HUH???" ?
        If InGroup("Pri-NL-Automatisering")
            $nul = AddPrinterConnection ("\\amssrv050\HP4000TN Administratie RAYE PS")
        EndIf

EndSelect


Allright ... the second case won't made it as the condition is the very same as in first case.
Think you need some reminder of what a select case statement does

Select case statements are mutually exclusive and processed from top: If the first case condition is met the script executes what is in there and continues right after the endselect keyword, all other cases will be ignored

hth
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