If a facebook group is "secret" (like the TDA).  If you just go to that page and nowhere else, you're probably ok.  However, if you use your personal home page & wall to post photos, info and your personal data for your friends to see, you may open yourself up to more "social" and ad messages than you want.  Here are the steps to decrease access to your personal information. 

 If you don’t want to use facebook as a social tool, just want to join a private facebook group, and are concerned about your privacy, just do this:

Sign up for facebook, provide minimum info required.  (Required to join our TDA group.)
Immediately open your home page.  (Click the link from the signup confirmation, for go to "facebook.com".
Click your name anywhere on the page. (Shows your profile)

To hide all information in your profile: 
Click "edit profile" (upper right corner)
Set each item to the privacy level you feel is safe -- the safest thing to do is to set them all to "only me".
Click "save changes.