Primarily used for someone visiting a patient in a hospital or a resident in a facility. Often appears in signs, rules, or formal announcements.
Visitors, please sign in at the reception desk.
The parking lot for hospital visitors is this way.
見舞い客 is a visitor who comes to see a sick person, similar to 面会者 but more casual and personal; 面会者 is more formal and institutional.
Compound of 面会 (meeting face-to-face) + 者 (person). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for forming agent nouns.