Prepare a spreadsheet document. List your players' names in the first column, and their e-mail addresses in the second column.
Save the spreadsheet in .xls format.
Once you've prepared the spreadsheet, go to your Project Manager page and click on the 'TOOLS' button on the left side of the page.
Click on 'Users'.
Click on 'invite more players'.
Under "Add Multiple Users", click 'choose file' to select the spreadsheet you prepared, and click 'upload'.
If you wish to notify your players about the invitation, check the box 'Send an email to this player'.
If the people you invited already have accounts, they will receive e-mails notifying them that they've been invited to the game.
If the people you invited do not have accounts, the system will create accounts 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.