Assigned User should autofill from domain's user database
Autofilling user emails from your google user profiles is now complete! To import your users, click on the question mark in the upper right corner and click “force sync”. Once it is done, please test out the feature and send me any and all feedback!
Ced Paine commented
To simplify and help ensure the accuracy of user assignments, typing the first few letters of a name or email address should bring up a dropdown list of matching domain users similar to Gmail.