This has changed in the Divvy 3.0 redesign and functions a little differently than before. Assigned team members are represented by their avatars in the top right of the Sandbox. To assign more team members to a content item, click the circle + icon to trigger the Add and Remove Team Members overlay. The list of users that you see are those people who can be assigned to the content item, based on the calendar to which it belongs. So, if you're trying to assign this content item to a certain person and they are not included in the list, you might need to check their calendar access.
Content Owner - (labeled as such in orange) Each content item must have a single content owner, which should be the person ultimately responsible for producing this content item, or managing the production process.
Assigned Team Members - Check the box for each team member who should be included in your workflow or approval process, or anyone who might want to keep tabs on progress. Assigned team members will then receive recent activity and/or email notifications when updates are made to this content item.