This is British slang, so you don't really hear it in America. And you're right, it does mean, "Who are all these people?" But it's quite informal. For example, if you were in a building and saw a bunch of government officials in suits, you probably wouldn't say, "Who are all this lot?" But if you walked into a nightclub and you saw a bunch of guys who were dressed poorly, you could say, "Who are all this lot?" Yes, it's grammatically incorrect, but, as you know, that's the nature of some slang. Hope this helps. Good luck!