Navigate to:
Dashboard -> Private Address Book -> Add Additional Organizations
If the Organization you wish to add is in the Public Address Book, you can pick it from the public listing.
If not, you need to add the Organization by entering the ID. In most cases, an organizations ID is the same as it's name.
When you send a supplier a link to register from the "Respondents" page in a project, the link looks like this:
http://demo.supplierselect.com/rfp/register/supplier/?buyer=Acme+Consultants
The last bit of this (buyer=Acme..) serves to add the newly registered organisation to the buyer's Private Address Book.