group is a name of the Windows NT group. It must be in the format Domain\Group. It is a sysname.
role is a name of the SQL Server role. It is of type sysname. It can include the database fixed roles or user-defined roles but not server roles.
Return type of IS_MEMBER function is an integer. It returns 0 when current user is not a member of group or role, 1 when current user is a member of group or role and NULL when group or role is not valid..
Examples of IS_MEMBER Function :