Try using the following options.
USE msdb;
CREATE USER ClarkKent FOR LOGIN ClarkKent;
exec sp_addrolemember 'DatabaseMailUserRole', 'ClarkKent'
, , ClarkKent , ;
USE msdb;
CREATE USER 'mydomain\BusinessGroup' FOR LOGIN 'mydomain\BusinessGroup';
exec sp_addrolemember 'DatabaseMailUserRole', 'mydomain\BusinessGroup'