It may be something as innocent as a member just left and has been trolling the group and they want to ensure that you are NOT that person coming back to do damage with such proof? But, this seems really wack to me, even if that is the case. You should be able to provide some bare bones info about yourself without your actual DL going over the net in email to anyone.
Identity theft can be done without a SSAN. With access to your DOB and your full name they can search a few data bases and find your criminal history and other personal information they do not need for you to be a member of their group. It is not a far leap to get your SSAN with your DL number and DOB and full name. It takes more time than if they have your SSAN, but it can be done. They can also set up a fake SSAN and go to work with your other info without even using your SSAN.
I would be wary. Are they all willing to scan and send their DLs to you FIRST? That would be telling. Although if it is a theft group, that may not be a problem for them as they likely have tons of IDs.
Just be careful and protect your private information and do what you think is best for you. Maybe you can find another crit group that will not require such information for membership?
Good luck!
Christine