If you're doing group membership lookup then no you would not need a UDF (a UDF is User Defined Function, please look in the FAQ section here on the board for further details on UDF)

If InGroup('MyDomain\MyNewGroup')
  'User Found' ?
Else
  'User not found' ?
EndIf
 
If you're basing it on machines then you will need to use a UDF for this.

Hopefully you're using KiXtart 4.23 or newer.