It's not possible to add a user with Service Desk application access (but not a project Agent) to the Customers list

Problem

It's not possible to add a user with Service Desk application access to a project's Customers list.

Steps to reproduce the issue

1. Create a Service Desk project with customer permissions to allow everyone with a Jira account to create requests.
2. Create 2 users (A & B) with application access to Service Desk.
3. Add user A to the project's Service Desk Team so that user A is an Agent of the project.
4. On the Customers page, try adding user A and B to the Customers list.
5. Create an Organization and add user A and B to the Organization.

Expected results

• User B is added to the Customers list while User A is not because User A is an Agent of the project.

Actual results

• It's not possible to add both user A and B to the Customers list, but it's possible to add them to an Organization.

Notes (Our expectations)

User A is an Agent of the project, it'd make sense that Agent does not appear in the Customers list but User B is not an Agent of the project, which makes this user a potential customer. It should be possible to have User B in the Customers list without even adding the user manually.