I was on Watch2gether with my friend on Tuesday, and this guy called username666 joined, without being given the link at all. It was already quite easy to figure out what could have been the reason because I think I gave another Steam friend the link to the room by accident before I gave it to my friend, and this random friend joined and saw we were watching spooky videos and changed his name to username666.
But today, a random guy joined again without invite, and we were watching stupid videos which weren't scary at all. He changed his name to kidboy, which made us laugh. Then he did a typo with username666 and forgot a 6, already showing he was a troll. He finally got username666 as a name, and put on that one "username666" video on Youtube. Best troll 2015. We just left and figured out that watch2gether probably lets the public join now.
tl/dr if you're gonna be a troll who wants to be scary, don't call yourself "kidboy."