sit in on — to attend a meeting, class, or discussion as an observer without being an official participant

a young professional sits quietly in a chair positioned slightly back from the main conference table, holding a notebook but not speaking. She watches attentively as the official meeting participants discuss around the table. The meeting continues with executives presenting charts and making decisions while she takes careful notes. Several suited businesspeople gesture and point at documents spread across the table during their active discussion. The observer remains respectfully silent, clearly present but not contributing to the formal proceedings. someone attending a meeting as an observer without being an official participant