Hi,
Is there a webhook that fires when I add a user to a group or approve their membership request? I tried the User Events (When a user logs in, logs out, is created, approved or updated) webhook but I don’t think it applies to groups. Also the Group Events hook doesn’t seem to trigger when I change members.
Hey! I’m trying to do the exact same thing. I’ll tell you if I manage to make it work.
I spent more time on it too and created a webhook that sends all available events. Nothing is fired when adding/removing members from a group, so I think we’re out of luck…
Indeed, I came to the same conclusion… 
I hope someone from the Discourse team will answer this question one day. 