OK, I've discovered that my problem , with the sp_AddRoleMember stored procedure, was using both [] and ''. What I've done is this:
EXEC sp_addrolemember N'db_datareader', N'DomainUsersAccess'
GO
EXEC sp_addrolemember N'db_datawriter', N'DomainUsersAccess'
GO
Kindest Regards, Rod Connect with me on LinkedIn.