Facebook – How to prevent friends adding me to their Facebook groups without the consent

It looks like this is currently somewhat broken.
If you go to an existing group (Aleph-One Strong for the Generalized Continuum Hypothesis!), then click on the category in the information, it takes you to a link which appears to be a search link (Outlandish Statements from the above group), but it gives you the message "Please enter a query." If you then put in a search term, it searches among all groups.

However, if you add &q=searchterm to the link, you get a search for your searchterm in your category. Taking the example from above, http://www.facebook.com/search.php?sfxp=1&c1=10&c2=141&type=groups&q=answer will get you a list of all groups in the category "Outlandish Statements".

With the millions of groups, and small number of categories for groups, a list of all groups in a category would be too large to look through them.

There is no way to access a group without a Facebook account.

To read a Group, you need:

• any valid access_token if the group is public (i.e. the group's privacy setting is OPEN)
• user_groups permission for a user's non-public groups
• friends_groups permission for a user's friend's non-public groups

The specific rules regarding groups and what is required to access them can be found in the Facebook Developers documentation.