Why do some Facebook pages/groups just need to add me from their side and I then immediately become joined it without my review first?
Some folks add me to very dirty sites and it appears in the group I joined automatically days ago.
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It's because that's how Facebook have designed Facebook Groups. See: https://www.facebook.com/help/147058305366686
It should only happen in Groups; people can't force you to "like" a page.
(If you have liked a page that you didn't actually click "like" on, then it's either caused by a misplaced click when using the mobile app/website, or else it's caused by a hacker that has created a web page that tricks you into clicking the Like button without your knowledge.)