To invite a new user to your project, start by going to your projects homepage, then clicking the admin button on the left side of your screen. Then on the right, open the User tab. Click "Invite User".
This will open a new window. Here you will enter the email address of the user you want to invite, then select a Role for them. Finally, you will click "Save" and the invite will be sent.
If you need to change a users applied role, you can do so by returning to the user page and selecting a new role from the drop down: