Well, Binkie, as our rules say, "many words regarded as dialect or specific to a particular country or region" are permitted, and most dictionaries list "youse" as regional or dialect, admittedly sometimes with the vaguely censorious note "non-standard". But in any case, we also allow slang and colloquial words. If there were no apostrophe in it, we would allow "y'all", a term which is probably on Biggerbirdbrain's lips on a regular basis. ("Did y'all have enough biscuits with y'all's breakfast?")
In fact, I think I've read some linguists say that such expressions are a means of providing a distinction between singular and plural you, such as many other languages have in their "standard" vocabulary.
But if youse disagree, I'd be happy to hear about it.
