The phrase "sit in on" refers to attending a meeting or gathering as an observer, without taking an active role. However, there are numerous synonyms that can be used in place of this phrase. For example, phrases such as "observe," "watch," "listen in," and "observe quietly" are all examples of synonyms that can be used interchangeably with "sit in on." Additionally, phrases such as "attend as a spectator," "participate passively," and "take a backseat" can also convey the same meaning as "sit in on." It is important to choose the right synonym that best suits the context of the situation to make it more understandable to the audience.