This might be the wrong category (sorry) but I have just gotten my group stolen because of a careless mistake which I take full responsibility on. I want to know how do I report it the group/the new owner?
Groups aren’t stolen because you can reclaim ownership within 30 days.
If that term has changed without me knowing, you can always report the new owner. Reporting the group might get it locked, but you won’t be able to recover it.
Your best bet is Roblox Support.