The IsMember method returns TRUE when the user or login referenced is a member of the role identified in the Role argument.
object.IsMember( Role )
object
Expression that evaluates to an object in the Applies To list
Role
String that identifies a Microsoft® SQL Server™ 2000 or database role by name
HRESULT IsMember(SQLDMO_LPCSTR szRole,
LPBOOL pRetVal);
For the Login object, the Role argument specifies a server role. For the User object, a system or user-defined database role is identified by the argument.