Over the past week I’ve done 4 events and have had trouble joining them.
2 of the 4 I got the prompt as normal to join. The other 2 I ended up quitting Zwift and restarting as it wasn’t giving me the option, and then the option only appeared after the event had started.
It seems like there’s no other way to join than the game to prompt you, there’s nothing in the App, and on the main menu although it lists the events on the top-right hand side and says “Going” your only option to interact with the event is to “Leave”.
Pretty poor User Interface Design.