Months and months before the event, there is a panel application that goes out where folks can pitch their panel idea to our Events team. Definitely keep a close eye on social media over the year to see that announcement go out - we're always looking for new ideas and fun activities!
As for participation in specific events, those can vary from panel to panel. Some panels have specific panelists that participate. And some events have contests or varying degrees of participation with sign ups (or "first come first serve). It really depends.
Wish I could be more help here! Let me know if you have any other questions!