I just called the Germantown dealership this evening and found out some more details. They said they would post information soon, but here is the gist of what they told me.
They do have parking arranged about a block away from the tidal basin. There is no need to sign up, everyone is welcome. There will be breakfast at the dealership from, I believe he said 7:30 through 9-ish, followed by a group photograph (at 9:15 I believe it was), followed by the drive to the cherry blossoms. I didn't have a pen and paper to write down the times when I called so I can't guarantee that I remembered them 100%- still that's the general plan.