There are probably good techniques worth stealing from FPS or MMORPGs. Those also usually run on a fixed tick rate of about 50ms, but can't afford to have people vote on it (FPS because lag is unacceptable, MMORPGs because of user count). There are various approaches in use, depending on whether you want to just degrade the experience of the lagging participant, or give preference based on action (e.g. FPS resolve conflicts in favor of the person who fired).