On the Project Manager page, click on the 'TOOLS' button' on the left side of the page.
Scroll to 'Users'.
Click on 'invite more players'.
Scroll down, and type in your invitee's name and e-mail address, then click 'add player to project'. If your invitee is an existing player, simply type in their e-mail address.
If you wish to notify your players about the invitation, check the box 'Send an email to this player'.
If your invitee already has an account, they will receive an e-mail notifying them that they've been invited to the game.
If your invitee does not have an account, the system will create an account for them, and they will receive an e-mail with a system-generated password to log in with, as well as a notification that they've been invited to the game.
This only allows you to invite one player at a time. See also: How do I invite multiple players at a time to my Project?