No it isn't. Once a 'race' is in progress any joining players have to wait until the next one begins before they can participate.
I didn't realise you were doing some of the coding now.
Anyway as I said elsewhere, there is no point adding this for the first release at this late stage. It was obviously a mistake, an oversight that didn't occur to anybody, and one that can be rectified in one of the many promised small releases/patches that have been promised to follow the initial release. I just hope it is hovvering near the top of 'Blokkers list'.
Well my Semi-Retired friend, I think that you've got a point there.