Not sure if this has been asked (I did search) but can an Xbox one party in Forza 5 join a hopper? We were trying to find a way for ages and in the end just gave up all trying to get in the same game.
If a member of the party is in a lobby then you will have the “Join Party Game” blue option appear in the lower left of the screen. Or if that does not display, go to Multiplayer and it will ask you there if you want to join game.