There are a plethora of synonyms for the phrase "sat in on" which refers to the informal act of attending or observing a meeting or discussion without actively participating. Some common alternatives include "observed," "attended," "listened in," "witnessed," "viewed," "watched," "monitored," "overheard," and "shadowed." These interchangeable terms can be used in a variety of settings, such as a college lecture, business conference, or legal proceeding. Whether you want to signify a passive or active level of participation, choosing the right synonym for "sat in on" can add variety and depth to your language and style.