You can register as either a circle or general participant on each event pages. Furthermore, you can join as many events as you like.
Circle Participation (Exhibitors)
If you'd like to attend an event as a circle, you can enter your circle information (your circle name, your niche, event tags, illustration IDs, and new publications) to advertise your content.
General Participants (Attendees)
You can let other pixiv users know that you're joining an event as an attendee.
You can change your participation status from the right side of the event page.
If you're registered as a general participant, you can pick between "leave event" and "switch to circle participation".
If you're registered as a circle participant, you can pick between "leave event as a circle" and "switch to general participation".