It would be neat to bypass the facebook group method and just have a website that you can request people's phone numbers (just a simple submit form). Then the numbers aren't publicly available for everyone to snoop.

The reason people make the group or event is because when you invite everyone they get a notification and a lot more people see it rather than a status update (that maybe <10% of your friends see). So maybe creating an event or group then linking to givemeyournumbers.com/john_smith