For clarity sake, there is no bus between T7 and T8 at JFK. Despite having so many OW airlines in the two terminals, and all of the many intra-alliance connections between the two terminals, there is in fact no connection between the two other than the free AirTrain and the public roadway (i.e. taxis), both of which are outside of security. Given that your flight departs from T8, you would not have a valid boarding pass that would permit you to clear security to enter T7.