I always liked being able to join custom races made by players to cruise or mess around, but was a little disappointed when I saw that It was gone on forza 5. Could they maybe add it in an update?
Build up your friends list, create your private custom lobby of choice and invite your friends. The non-public part obviously makes it difficult to meet different gamers, but its likely as good as its gonna get on this title.
i would also post it here where T10 are likely to see it instead of making yet another thread thats ganna get locked and then never seen by T10