I believe in any corporate setting you can offer incentives to those using their personal time to benefit their health. Incentives can include anything from a competition, to earning raffle tickets each time you go to the gym.
Another thing is that giving up food at lunch time in exchange for workout time can be difficult. So maybe you can increase the rate slightly and offer an option package that includes a health lunch pack, smoothies, etc as part of the lunch workout. You might have to start out with a small group scale at first and run experiments.
Stay accountable as to what works and what doesn’t. If fitness facility does not have a gym management system to track these experiments, you might want to consider getting one. Remember, “what gets measured, gets improved.”
For more information on a gym tracking system, go to: