To manage your team, click on the "Team" button on the top right of your Project.
To add a team member, click "Add Member" and enter the email address of the teammate you want to bring in to this Project.
You can set whether they are an admin or not by adjusting the admin toggle. To choose which permissions admins and users have, check out this article.
Now when your team member refreshes they will see your Project inside their dashboard and you can start collaborating immediately :)
To remove a member from your team, you can click "Remove" on the right side of the team management area.