Master Domain
Resource Domain (one-way TRUST to Master Domain)
NT Workstation and Win9x Clients

I have added output of @ERROR to the test script for each INGROUP operation. The results are below.
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C:\Data\Scripts>kix32 test.kix

HomeDrive = H:

ResourceDomain\Account Operators return code = 0
Err: 1355
Is a member of Pipsqueak\Account Operators
Err: 0
Err: 1355
C:\Data\Scripts>net helpmsg 1355

The specified domain did not exist.
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I have added a trust from the master domain to the resource domain thereby creating a two-way trust. The result is the same.