Often used for someone visiting a school, factory, or institution to observe, rather than a casual guest.
On the school open day, many visitors came to the classroom.
訪問者 is a general visitor, often for social or business calls, whereas 参観者 implies observing a place or activity.
Compound of 参観 (sankan, 'visit for observation') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the kanji spelling is conventionally associated with the meaning.