If you haven't created another group to send the invite to, you'll need to go to the Groups area first to create your group and add email addresses. To find out how to import email addresses, click here. Once your group has been created and emails added to the group, go the left side of your account Sign Ups page and click Messages. Select the Compose tab and then Invite people to a sign up. Select a sign up and click Select People in order to choose a new group to send the invite to.