With YouTrack 6.5 we introduced the concept of a project team. A project team is created automatically while creating a new project, and by default contains the project's administrator — a user who created the project is granted the Project Admin role in this project.
For more information about project team please see this article.
Project Team vs Project Assignees
Project Team is is a subset of project Assignees. The Team represents an actual team of an actual project. And like in any real-life project, a user might not be a member of the Team, but still can be a potential assignee for issues in the project. In terms of YouTrack, Project Assignees is actually a set of issue field values containing the list of users who can be assigned an issue in the tracker. Please refer to the Managing Project Assignees doc page for details on configuring the list of possible assignees in a project.