Since a few months at least I notice that when I react in a FB group to one of the postings that my other friends then like my reaction there, even though they aren't a member of the group.

I don't like that, I feel it ruins my personal freedom and privacy, I don't need everyone to know where I hang out and what I say.
Is there something I can do about this in privacy settings? Or does this have to do with the privacy settings of that group?

I tried to Google it, but not sure how to phrase the question so it didn't help.
I think it's only with groups.

Hope someone here knows!! Thank you.

You're better off getting as far away from Fakebook as you can.

And just think how many saw what you posted and didn't respond.. ..In the shadows (insert heavy breathing here)..watching..lurking..munching on popcorn..while commanding another to (heavier breathing here)(almost passed out) ..to..rub the lotion on their skin..or else they get the hose.. ..what were we talking about?..oh yea facebook..yea stay away from that..lurker central..I don't go there.. ...too many weirdos..LOL

That doesn't answer my question...

I did. You just didn't like the answer.

I think, it is your own privacy settings. But I only know about the option, to exclude friends from viewing a certain post. Share with all friends, except (insert name of friend you wanna exclude), something like that

Yes, that's about as far as I got too, hihi. Maybe it's with notifications as I do think people get a notification of me posting there.
Come to think of it, if a FB friend posts or replies on a FB group I also get notification. NOT when they post on another person's FB, only the groups.
Weird.

I have perfect FB security.
I don't go there.

You should talk to your group administrator about this. He/she should be able to help you with this. FB has started cracking down on a lot of things. Some things still slip through. If you feel it ruins your personal freedom and privacy, The group administrator should be your first go too.

FaceBook, is nothing more than a phishing scam,
to collect and market the personal data of it's members.

Yes I think that's what it comes down to. I indeed don't like it, it does ruin my personal freedom. Thank you!

I don't really care to be honest. If you start thinking that way, you might as well do away with your PC, your phone -including land line-, not go outdoors anymore at all as there are drones everywhere and satellites and cameras in the streets.
But your also not safe indoors from spying eyes & ears as when it comes to the crunch government has equipment with which they can see through walls, hear through walls and so on.
Even your neighbour can now buy quality drones, allowing them to spot everyone in their own backyard.
Don't drive a car anymore as that can be tracked when it's newer.
Don't use a bank card or credit card as that can be tracked...
Then you might as well leave the planet. Unfortunately that is impossible, and would also require equipment that can be tracked.