Click your profile picture on the top right corner and choose Team and Roles.
By creating the project, you are the administrator. Administrators have complete access to the project, including adding new supporters.
To add a new supporter, click New user in the Supporters panel.
You can add a supporter using their email address, Facebook ID, or Google ID. Press enter after each email or ID to add multiple supporters at a time. Enter the Role for the supporter(s).
Check the checkbox for the permissions you want your newly added supporter(s) to have. You can always change this later.