Sometimes in speech, when someone is trying to be playful or coy, they might use the word "we" to just mean "you", almost always beginning with "aren't we ___". I don't really know when this started, and it isn't grammatically correct, but people sometimes do it. When he says "Aren't we Mr 'Glass half full'", you are correct, he is only referring to Ross. For example, if you meet your girlfriend for a date you might say "Well, aren't we looking lovely today?" So its used to be playful, but be careful, as I think it can also come across as a little patronising. Like you're talking down to someone a little bit.

Well, you're right in your assumption. I feel, aren't we isn't used too commonly in the U.S, so there really is no specific usage, as far as everyday conversations go.

When he says "aren't we" he is referring to himself, Joey and Ross.