You may want to check this Friday's edition of the Panorama to see if they publish information about the bus. Assume that it is a US-sponsored bus ride as I doubt the German delegation is footing the bill.
As for sponsoring, it is a crap shoot and depends on who is manning the gate, and I've had so many different experiences (using NATO ID, US ID, etc) that I couldn't really tell you what the policy actually is...sorry.
Plus, I'm not the military member, but the spouse for whom a whole other set of rules apply...as always. :)