There are two ways to add projects to a Team.
If you're in a Team, you can add your projects from the Team page itself using the Projects drop down, then checking "add to team page" next to projects you want to add
(Note: only Admins will see the Members and Edit Info tabs, but the Projects functionality is the same)
Alternatively, you can add a project to a team from the Project Editor. From the editor, click 3: Settings in the top left, then Discoverability from the next popup, and then Teams. Click Edit next to Teams to choose the Team(s) you'd like to add the project to. You don't have to be a member of a Team to add your project to the Team, but an admin will have to confirm that the project was created for their Team before it will appear. Click Done to save your changes.
Note: only public projects can be added to Teams