It might be a good idea to mention your concerns to whoever runs the session, if you can get a chance to have a quiet word before it starts. Then maybe he/she can make a general announcement to the effect that people should keep quiet while others are performing.

That assumes, of course, that the person isn't one of the three 'offenders'; and that it is indeed the etiquette to keep quiet while people are performing (that seems to be the case normally, but there may be exceptions).