Navigate to the Group Info tab

In the Trip Roster, use the dropdown next to the user's name and select Resend [USER]'s Trip Invitation. As long as the user has a valid email address, they will receive another invitation with instructions on how to access their account.

To re-send multiple trip invitations, select the users and use the Resend Trip Invitation button on the left. As long as the user has a valid email address, they will receive another invitation with instructions on how to access their account.