You can import contacts from your email program or other contact tracker. You'll need to first export the contact list as a comma-separated values (CSV) file from your email program. Most email programs give you the ability to do this.
Once you have the contact list as a CSV file, go to your group's main page and click Invite people to join this group. There, click the Import contacts link and browse for the CSV file.
If you want to delete or resend invitations, you do so while managing group membership. This can be handy if you've got a lot of outstanding unanswered invitations. Go to your group's main page, then click Manage group members. On the Manage group members page, click the Open Invitations link and select the names of people whose invitations you want to delete or resend. Use the dropdown above the list to select the action you want to take.