From Books Online: "'login' is the login name of the user. login is sysname. login, which is optional, can be a Microsoft SQL Server login or Microsoft Windows user or group. If login is not specified, information about the current security context is returned."

What does the following give you?

SELECT SUSER_ID(N'test\userA');

SELECT SUSER_ID(N'test\groupB');

SELECT sid

FROM sys.server_principals

WHERE name IN (N'test\userA', N'test\groupB');