To join a group, go to a group and click Join Group in the top-right corner.
Depending on the group's settings, you may have to wait for a group admin to approve your request.
Keep in mind when you join a group:
+ People may see when you join a public or closed group or that you're a member of that group, like in News Feed or search.
+ When someone adds you to a public or closed group, others may see that you've been invited (ex: in News Feed or search). Once you visit a group you're invited to, you'll become a member and other people may see you joined.
+ Certain groups on Facebook have their privacy setting set to secret and may not appear in search results. You can see them once a group member adds you.