If supplier self registration is enabled, as soon as the supplier is created in SUS there will be two emails sent out with user name and password. Supplier can use this data to login to the srmsus_selfreg service and create his own SUS administrator user. Once this is done supplier can logon to SUS and create as many users as he wants via the user admin UI in SUS.
If supplier self registration is not enabled then a purchaser administrator will need to create the admin user for the supplier manually and provide user name and password to them.
regards
Zoltan