Yes, you can invite non-Facebook users into your group, but they will not be able to join until they register for Facebook. If you go to the "Members" tab of the "Edit Group Profile" page, you will see a section that allows you to "Invite People who are not on Facebook via Email." If you list your friends' email addresses, it will send them a group invitation. In order to join the group, however, your friends are going to need to register for Facebook. It will explain this process to them in the email they receive. This question was asked on the wiki: Facebookstalker Wiki.