("sich+Acc mitsetzen" - technically, the word can be used if someone decides to sit down as someone else does it. It's NOT for sitting next to someone... just for sitting down at the same time as some sort of "joined activity". Aka.... NOT COMMON ;)!)
2.
to also bet, to bet along
(Either placing down a bet along with someone or placing down some stake along with other stakes. Think of someone betting his Ray Ban along with the money. Not as rare as the first meaning but still... not really something to "learn".)
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We haven't talked about this word in an article in detail. Just leave your questions in the discussion section here, and I'll answer all them :)