They reroute you to an alternate location. In the case of the Grandeur, a friend of mine was aboard and his muster station was one of those that was not safe to use. He said crew were stationed in the paths leading to his station both telling people to go to the main theater (their alternate muster area) and providing directions on how to get there. I think he said some crew also escorted groups to the theater as people gathered. I heard another passenger on a radio show link who said they also had crew physically located at the closed stations to redirect any guests who had managed to get there despite the roadblocks along the way.
My friend said despite being rerouted he was seated in the theater less than 10 minutes after the general alarm was sounded - and that included waking up, getting dressed, etc.