As noted by Jay Roberts of Alexandria in his Jaybird’s Jottings blog, the Open had shuttle buses from Tysons Corner and Northern Virginia Community College in Annnandale when they were last in Bethesda, in 1997. This time the only shuttle lots are in...Gaithersburg.
If you want to take Metro, you must reserve and pay in advance for a shuttle bus from the Grosvenor-Strathmore station. A USGA official told Roberts there is a shuttle from Dulles, but that’s not in the official brochure for fans, so I don’t know how anyone would know about it.
Start planning now, duffers.