There are three ways to invite people to a Pay By Group.
1) The "Invite Friends" button will let you put in the email addresses of your invitees.
2) Each Pay By Group URL is unique. Sending someone a link to your specific group's URL has the same effect as inviting them to the group directly.
3) Posting the link via social tools like Facebook and Twitter allows you to quickly invite large groups of people to join your group.