sit in on something

No, sitting in on something is when you're either learning how someone does something or you're appraising them in some way. You're just going to the lecture. You would be sitting in on it if you were about to take over his job, or if you were to submit a report to his boss on how well he performed.

Sitting in can be quite hands on, the students may ask the lecturer a question, and if the person sitting in on the lecture knows more about that particular topic than the lecturer does, he may ask the person sitting in to answer it.

Yes, you can say that for, it appears, both NaE and BrE, and I suspect AuE and NzE too.

'sit in on sth' means to take part in something that one is not a regular part of.

=====================

verb
sit in intr, adverb


4 foll by on to take part (in) as a visitor or guest ⇒ "we sat in on Professor Johnson's seminar"

http://www.collinsdictionary.com/dictionary/english/sit-in

=========================
sit in

: to take part in or be present at a session of music or discussion as a visitor —often used with on <invited to sit in on a rehearsal>

http://www.merriam-webster.com/dictionary/sit-in?show=1&t=1380136988

=================