Sit in on = to be a spectator, observer, or visitor at: to sit in on classes.
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I am wiritng this in 2019. Lol I can't beleive you posted this in 2009 and never got a reply. Becuase I am here wondering the same thing.
BamtoriI've known this phrasal verb from this expression, " You'll have to sit in on all of the meetings with them.". I guess "sit in on a meeting" means to attend a meeting, right? With the preposit