With our 150+ MB savegame, the hosts PC freezes for 5 to 10 seconds while performing autosaves. In Multiplayer, joined players do not notice this time before after it happened. You can still move around and build/destroy things, however, they won't be placed or removed in this time. After the save, sometimes crazy stuff happens like buildings being built not where you clicked or buildings are build but not visible to the joined players until he leaves and rejoins the game. Last weekend I had to rejoin the multiplayer session about 3 to 4 times per hour to efficiently build my base.
I would suggest that the host sends a signal to the client, which will show some autosave animation in one of the display corners (like it is done in many modern games), 3 seconds before the game actually saves. While this animation is played, currently active player actions like building and destroying should be cancelled and players should not be able to do one of this things at all.
After the host saved the game, another signal would be send to the clients to stop the animation and players can go on building.