Customer users cannot share tickets they created

Francisco Esteves_6vp0y1

Customers get the following message when they attempt to add someone on a help desk ticket.

The field allows for the option to add, but when pressing the "add" button it presents the message seen above.

Based on the documentation for customer sharing I think I have set everything correctly. Below are my customer portal configurations:

 

Jack Brickey

Are they attempting to add a user that is in their Organization as defined in the Customers page within the project? Your current sharing restriction is that they must be in the same Organization.