What's "count in" mean? Is there a "count out" too?

Native speaker’s answer
Rebecca
To "count [one] in" means to include someone in something or to say that you'll join or be a part of something. There is also a "count [one] out"! It means to exclude or not regard as part of something. This is common when making plans with more than one person or after something is suggested. "Count in" can also mean to literally count so that someone can start a song with singing or an instrument to make sure everyone starts at the same or so that it's on the beat. Ex: Count me in for lunch this afternoon! Ex: Count me out for drinks this evening. I need to go to sleep early. Ex: Can you count me in for the last song?