Have you noticed that it’s common for people talking about events to refer to those who come along as attendees?
The “-ee” suffix means something happens to you. For example, an employee is someone who is employed by another person or organisation. Attendee actually means someone who is attended – perhaps they have servants!
To talk about someone who does the thing, you need “-er” – so employer or attender.
Slight snag: attender sounds awkward and impersonal when you use it. “The conference had two hundred attenders.”
Maybe we’d do better with a different word entirely. You could try delegate, especially if you want to sound a bit posh and official, though it really means someone appointed to represent another person or body, not just someone who turns up.
You know what you could call people who visit your event? Visitors.