In Hong Kong, 旁觀者 means bystander. So I would say most people in Hong Kong would not use 旁觀 in this situation.
旁聽 is a more commonly used word. However, if the sit-in is meant to be a try-out, then I think 試堂 (試 /si3/ as in 試驗) would be a more accurate word.
旁聽 is a more commonly used word. However, if the sit-in is meant to be a try-out, then I think 試堂 (試 /si3/ as in 試驗) would be a more accurate word.