When you start zwift, on the right hand side of the screen should be "Upcoming Events". Scroll through, click on what you want to sign up. Alternately use zwiftlink to add yourself to an upcoming event. I prefer zwiftlink as I can sign up for events many hours in advance.
Once signed up, start riding in zwift. Shortly before the event, a button will appear in the lower left part of your screen asking if you want to join the event (IIRC it even has a timer). When you want to join, click it and you'll be taken to the holding area. You can continue to warm up in the holding area.