Does the phrase "who's in?" or "I'm in!" exist in (informal) English?

The phrase "Who's in?" does exist in very informal English, at least in American English. It is equivalent to saying "Who wants to participate in X with me?" It is not used very often, at least in my experience. However, people will understand what it means if you say it in conversation. For example, if you wanted to get food:

I'm feeling hungry, so I'm going to order pizza. Who's in?

Yes, these phrases exist.

Also "I'm out" for declining.

"I'm in and (more often) "I'm out" are used a lot in the British television programme Dragon's Den where, after hearing a pitch, investors decide whether they wish to invest or not.

"I'm in" is an alternate form of "count me in" which means "include me". This is similar to the poker phrase "deal me in", but I don't know which came first or if one was derived from the other.

Similarly, "I'm out" is an alternate form of "count me out" which means "exclude me". On a related note Dictionary.com says that "count out" is also a boxing term referring to when a boxer loses because he has been knocked down and can't get up before the referee finishes a 10-second count. I would guess that the boxing term was developed independently of the "exclude me" meaning.

I'm Italian, I don't remember if I learned this in school or by seeing movies/tv series, but I always believed it was a common way of saying, that it was used normally. I don't find it old at all! It's funny to read that you don't use it that much...