These are part of the system webhooks.
And yes, these get fired when a user is added to any social group as member or his membership is removed.
This does not handle the follow/unfollow events.
Thanks for your reply Pawan!
is the webhook even fired if the role within a place is changed (e.g. from member to administrator) or if the user is banned??
To be frank, I had checked the effect if these transitions on webhooks long long back.
I am not able to recollect it now.
I think change from member to admin will not trigger an event.
Banning should ideally trigger, though banned is still a membership stage. Need to check this again. Sorry folks