First thing to note is that English is an ever changing language with many different dialects, one of which is taken to be "standard" or "correct" English. However, words from regional dialects or newly coined words cross over into standard English all the time.
Since the words "thou" and "ye" fell out of usage in English for singular and plural respectively, the word "you" has gained traction as both the second person singular and the second person plural. This leaves a gap in the language though, and "pressure" to fill the gap. Speakers of languages including this distinction often find it frustrating that English lacks it, and indeed, even monolingual speakers of English run into times where the lack of specificity of "you" becomes a pain.
Just yesterday I was writing an Email to someone in a department of a company in another part of the country, as it had been indicated to me that his dept. was responsible for some process which I had some questions about. The email I wanted to write was:
"John tells me he thinks youse look after [certain process], could you assist me with X Y & Z".
Unfortunately, because of the stigma of using a word like "yous", I was left wanting. If I had said "you", the person reading the email would have taken it to mean that he personally was responsible for the process, which I am positive he isn't. I could have gone for "you lot", but that sounds a little rude to me (Oi! You lot!). In the end, I caved and wrote "you guys", which doesn't feel at all natural to me, a Brit, but filled the gap I needed.
As usually happens when language shifts to lose a useful piece of grammar, dialect variations that cover this gap tend to spring up. The most notable of these are:
- Y'all chiefly found in southern American English and African American English (interesting article about it's propogation here http://dialectblog.com/2011/02/15/the-remarkable-history-of-yall/)
- You guys as increasingly found in General American as a generic second person plural. The fact that people use it even for a mixed gender or all female group implies a certain level of grammaticalisation of the form.
- Yous/Youse is what's used around my neck of the woods. I believe it's used in some parts of the states too. In Boston this seems to have come from Irish immigrants, but it being formed just by adding an s to you, it wouldn't be surprising if it had sprung up independently in many places. Scouse (Liverpudlian) English uses it, likely also a borrowing from the Irish, who often pronounce it closer to "yez" or "yiz".
An interesting article about all of this here, if you're interested.