Hi Max,
In Case of MM-SUS scenario, we will initiate an Interface from SAP to SUS to create first time User ID, usually this user ID will have Admin access for supplier company.
Post creation of this user they will create separate user ID's for their different user base like Sales, Production, Engineering.....
When they create ID's it will get Mapped with Admin user. So for Customer Company admin user ID will be primary point of contact & all the communication will be send to this email address. As far as i know this is SAP's standard process.
In case of your customer requirement i think you need explore customize options.
Thanks
Manish