You can add team members in the "Team" tab of the project editor.

Click "Add a member", and then under "Select a user" start typing the other member's name or user name. When you see their name appear in the list, select it. Don't forget to click "Save changes" at the bottom to finalize your changes.

The project will be owned by the collective “Team”, therefore anyone who is listed as a manager is essentially an owners of that project. A "team" always owns a project, even when there is only 1 user. The initial user’s username is used in the url as a default. If a team name is created after another user is added to the project, then the project’s url will update.