sit in

sit in  {v.}

1. Быть членом чего-л., принимать участие.

We're having a conference and we'd like you to sit in.

also sit in on: To be a member of; participate in.

We want you to sit in on the meeting.

2. Посещать, но не принимать участие. Часто исп. с "on".

Our teacher was invited to sit in on the conference.

Ср.: LOOK ON.