UK: In addition to the other answers, if we were meeting two people we'd probably say "How are you both?". More than 2, '"How are you all?" or "How is everyone?". If you are meeting just a few people, say 1 - 5, you might say, "How are we?", we meaning singular or plural you!

In the UK, we don't really use 'y'all', but we do use 'you all' and 'you guys'. I would say that these are the most common. In some dialects in the UK and Ireland (perhaps in Australia as well), people say 'youse'.

Y'all is only used in the south in the US. The grammatically correct plural of "you" is still "you". In colloquial speech in Canada we use "you guys" but not in formal speech or in writing.