It's fine to confirm the participation to the game just before it starts. However it will help us with choosing the best possible settings for the map if we have a better idea about the number of the players joining the game. You can of course change your mind and remove your confirmation as long as you do it before the game starts.
We will probably start the next test game with (mostly only) confirmed players. This should give us a better idea about the actual map size we will use for the game.