Create a customer named ACME CORP
Now create a Person who is associated with ACME CORP.
The Cust/Vendor field is farther down on the page.
Now create a user that is associated with that Person.
Now create a Security Group (SP) with any permissions you want, but specify "Authorize Group for Customer on User's Person record". I only granted Read access to the Assets application. And add your user to this group.
Here I'm logged in as the user, and can only see the one asset associated with ACME CORP.
Here I'm viewing the Licenses (SP) application for Adobe Acrobat and see that a license has been allocated to ITAM1010, which is the asset associated with ACME CORP.