Krux wrote:it is not possible to join a running game, because the engine wont't support it.
I have read, that only input is sent over the net, this makes it hard to join a running game, because the new player doesn't know anything about the others.
it's not exactly input but that's close enough to get the point.
it would be conceivable to freeze the game while a new player downloaded one of the automatic game recording from the host and run the simulation forward to the current game point to sync up... and someone was working on a "re-syncing" version of spring a while back and there is work towards of being able to load a saved game for a group of players hiding incomplete in the engine code... but the practical implementation of this feature is quite a lot of work from what I understand so has not been completed.
For the time being there is little difference between spectating a live game and watching a hosted replay so that's probably your best option.