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. โ€ฆ

Jul 4, 2018ย ยท The expression "I'm in" or "count me in" mean that you wish to be included in a proposed activity. For example: "I'm going to the bar. Anyone else coming?" "Count me in!" I believe the โ€ฆ

Mar 5, 2018ย ยท According to the Cambridge dictionary, on it means informally doing something that needs to be done, or trying to solve a problem. In my experience it doesn't imply one or the other. In fact, โ€ฆ

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