You can only be added into groups without your permission by existing Facebook friends. It’s quite possible they added you with good intentions, but if they are continually adding you you’ll need to take action by either asking them politely to stop or removing them from your friends list altogether.
The only way you can really find out who added you is if you were added recently, in which case you should be able to find out from the notifications by clicking on the globe icon at the top. Look for a message that looks like the one below:
If not, you’ll have to keep an eye out in future. You should probably tell anyone adding you to groups too frequently that you’d rather they didn’t, and if they continue even consider removing them.
Step 3 – Reducing group posts from your News Feed
There may be some groups you like being a part of but would like to see less of on your News Feed. Thankfully, there is a solution for this too.
Go to the GROUPS page once again and click the cog icon next to the group in question. Select Edit Notification Settings.
From the drop-down box, select how often you’d like to be notified about updates in the group. You can choose from All Posts (so every post), Highlights (which is the default setting), Friends’ Posts (only a notification when a friend shares something in the group) or Off altogether. Then hit Save Changes.