This can be done by selection of the correct specialization.
In my test project I have an entity for which I store the owner attribute:
I created an overview page with a datagrid showing the Attribute of the entity. I added a second column on the right and from the connector panel I can select the owner:
When the owner is expanded I will see the user entity and the specializations, in my case just one: Account.
From the account I can select the FullName attribute to be displayed in the column:
The same option is available in the dataview used for adding new records and editing them.
Be aware that when a user that is not an account, but created directly in the user entity will not have the fullname available and the field and column will then show empty. As long as all users are created as accounts the fullname can be displayed as described above.
Note: It seems the images are not being displayed correctly, I have send feedback to Mendix on this.