How do I join a group?

Facebook answers the question of how to become a member of a group in the Causes Help Center.   Their answer:

You can join any open group that is set as global or that is limited to one of your current networks. You can search for groups by typing a name in the Search field at the top of the page, and then filtering the results to show only groups by clicking the Groups tab. Alternatively, you can browse groups by category from the Groups main page. To become a member, click the group's name, and then click "Join this Group" under the group picture on the right side of the screen.

Some groups require admin approval for you to join. If you try to join these groups, you will have to wait for an admin to confirm your request before you can join the group. Admins can change the settings for who can join a group at any time, and can also block specific people from joining a group.

Certain groups on Facebook require an invitation in order to join. You cannot request to join these. Only an invitation from a group admin will give you access.