The article outlines the steps to allocate an external user to a vendor. (Please note to perform this action you require the "manage customers and vendors" permission)
FOLLOW STEPS BELOW:
1. Via the Navigation pane click Administration->Vendors
2. Double click on the vendor you would like to allocate an external user to.
3. Click and open the external user tab.
4. Click the checkbox on the left of the username field to allocate this user to the vendor
NB. For a quick tutorial watch below
