You can't prevent people from adding you to groups - you can only leave a group once added.
See this Facebook article
Can I prevent people from adding me to a new group?
The functionality of approving a group
membership is not available. Similar
to being tagged in a photo, you can
only be added to a group by one of
your friends. When a friend adds you
to a group, a story in the group (and
in News Feed for Open or Closed
groups) will indicate that your friend
has added you to a group.
Please keep in mind:
- Only your friends can add you to groups.
- When a friend adds you to a group, you'll get a notification right away.
- You can leave a group anytime. To do so, just go to the group page and
click "Leave Group" in the right-hand
column. Once you leave a group, you
can't be added by anyone else unless
you explicitly request to be re-added