Значение идиомы sit in

[sit in] {v.} 1. To be a member; participate.

We’re having aconference and we’d like you to sit in.

Also [sit in on]: To be amember of; participate in.

We want you to sit in on the meeting.

2. To attend but not participate. Often used with “on”.

Our teacherwas invited to sit in on the conference.

Compare: LOOK ON.