How can existing contract users not created as resources get business unit access?

Existing contract users who weren't created as resources can get business unit (BU) access if you make them resources by using the Identify Resource feature.

You can use the Identify Resource feature to search for a user and identify the user as a resource through the Add as Resource option. While adding the user as a resource, the user can be associated with a resource organization. Once a user is mapped to a resource organization, the user gets access to the BU mapped to the resource organization.