Facebook groups give anyone who has registered a Facebook account who is a member of the group to arbitrarily add any other Facebook user to a group without asking first as long as that user is on his or her friends list.
Whether your addition to a group by someone on your friends list was meant to benefit you or was done maliciously, you aren’t given the opportunity to opt-in.
You are in.
What Happens When You Are Added to a New Group
All groups require member approval by either an admin or another group member, depending on the group’s settings.
In the case of public and closed groups, anyone can see the list of members of the group, its name, and topic. In secret groups, only current members of the secret group can see the member list.
When you are added to a new group, Facebook sends you a notification.
Click on the Groups listing at the left of your newsfeed and locate the new group.
Click on its name to go to the group page.
If you aren’t interested in being in the group, you can immediately opt out by clicking the Joined button and selecting Leave Group.
After you leave a group, you can’t be added by anyone else unless you ask to be re-added to the group.
If you decide to remain in the group, you’ll see group posts in your news feed unless you select the Unfollow Group option, also under the Joined button on the group’s page, and you can post to the group.
How to Prevent Friends From Adding You to Groups Without Permission
There’s no way to prevent one of your Facebook friends from adding you to a group, but you have a few options to prevent it from happening a second time:
1. Tell the friend directly that you don’t want to be added to groups without your permission.
2. Block the friend.
3. Unfriend the friend.