Once you've selected the name of the main plan artists on a release, you can add a featured artists too.
This works the same in both the Ditto Dashboard and Ditto Mobile App:
Below the primary artist heading, click the toggle 'Would you like to add any Featured artists to this release'.
Choose an existing artist on your account or choose to Add More Artists and add a new one. Then just enter the name of your featured artist.
On the Ditto Dashboard:
On the Ditto Mobile App:



