An Attendee

1. A person who is in attendance or in the audience of an event.
2. A visitor or participant of an event.
3. (uncommon) A person who is attended.

(participant in an event): audience (member), participant

(person in attendance): attender, audience (member)

Attender was originally the only word for a person attending. [1] As with most nouns formed from verbs, as payer, trainer, employer, it was the receiver of action that was formed with -ee, as with payee, trainee, employee. In the 1980s with the advent of spell-checkers, the word attender was erroneously flagged as misspelled and attendee was its replacement. Since then attender is no longer in popular usage.