I called Grassley's office.
They say that you can just show up at his Cedar Rapids office to get tickets.

You can pickup tickets from Sen Grassley's office until 5 PM on the 29th.
Or show up at 8 am on the 30th (presumably they will be there until at least 9?)

Grassley's Senate office:
Cedar Rapids
206 Federal Bldg (Between 1st and 2nd Ave)
Enter on 2nd ave Entrance.
319-363-6832

Doors to the event (at Kirkland Comm college) open at 10 AM.