What is the best way to motivate employees and managers to participate in safety committee meetings?

There isn't a direct way - nobody wants to go to meetings like that.  But if nobody is talking, you may have additional problems.  It's possible that everybody is scared of speaking up. 

Keep the meetings short and publish the agenda in advance.  Have them at a time and place that helps people to participate.

If all matter & total circumstances are control patience & within short time program, they may be attend.