We highly recommend that SIY programs (especially the 2-day program) not be made mandatory, and that participants can opt into the program fully. Since the SIY program can be personal in nature, for full participant buy-in and the best outcomes (and best teaching experience for you), participants should feel like they have chosen to attend.
The keynote format (and occasionally the half-day) can be an exception to this since it's meant to be a short introduction, and can work well as a presentation during a mandatory staff offsite or event.
If a program is being run for an intact team, the team lead might still give people the opportunity to opt out. They might share with the team why they think that the program will be valuable and strongly encourage attendance, but offer a chance to opt-out.
If you are teaching an SIY program that is required, please emphasize in the ground rules section that participation in any given exercise is at their discretion.