Image I go to my friend's university and join one of their class(by join I mean just sit in the classroom and listening to the lecture)
Is there a phrase/word for this type of behavior or situation?
Word Usage – What’s the Word for Joining a University Class When Not a Student?
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Best Answer
The phrase to use for informally joining a lecture, meeting, etc. is "to sit in on".
The phrase is quite idiomatic and common among native speakers.
As Ross mentioned in another answer. When there is a formal arrangement to attend a class without necessarily performing all the work or receiving a grade, the phrase would be "to audit".