Maybe you did not put the computer in the group or the group is not a security group. I tested the UDF with my XP computer on KiX4.02 and 4.22. I don't have 4.11 because it was a buggy build.

Is that the whole code? You could put a few lines in to echo to the console.
Code:
If ComputerInGroup ("PC-CENT-SI")=1
 ? 'UDF ComputerInGroup works...'
else
 ? 'UDF ComputerNOT InGroup...'
Endif  
'Error #'+@Error+' ['+@SError+']'

sleep 1000
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