How are you determing whether this portion is returning a value of zero? Does the script resolve the group memberships properly in other ways (drives mapped, etc), but appears to always return a value of zero?

For example, if your script is:
IF INGROUP = ()
IF @ERROR = 0

Using '@ERROR' on "IF INGROUP" will properly return a zero every time if the "IF INGROUP" script is correctly formatted and executed.

If using a variable substitution, such as
$RC = INGROUP ()
? $RC

Then the variable $RC should properly return a value of 0, 1 or 2 depending upon group membership.

Hope this helps.

Bill