The phrase "sitting in on" refers to participating in a meeting, class, or event as an observer without actively engaging. There are several synonyms for this phrase that can be used based on the context. The common synonyms for "sitting in on" are attending, observing, monitoring, and watching. Other synonyms include sitting through, sitting out, and sitting quietly. These synonyms are used to indicate being present at an event or class, but not necessarily participating in the discussion. While these phrases are used interchangeably, the context and intention of the speaker should be considered to avoid confusion.