How did someone join my Map when i wasn't in a party

So today at 9:49am Central time I was just doing a LVL 66 Tropical map and enjoying a solo play. Towards the end some guy named "Tanthrax" Runs in screams out It's a Bug and starts taking crap. At the end i was trying to ask him nicely to leave and he brings over boss and I die as typing and he takes all the loot. I'm not sure if this is in the right place to report this but how did he even get into the map? This almost made me uninstall the game the very instance it happened I really hope there is a solution to this problem. Also when I right click his name it doesn't give me the option to report him.

Thanks

nm when I went back in the map was still there.... very strange