The THV gate is not manned. You need a room key to get in. I'm not sure if there's an intercom button. However, I seem to recall when I got a taxi from SSR three or four years ago, we had to meet them at the carriage house. They weren't supposed to come directly to a specific room number and wouldn't pick us up in the Paddock. Your experience may vary.
Last time we stayed at the THV, my parents joined us later in the day after we'd checked in. They called me when they were close, and I walked up to the gate with a key and let them in. After having done that, and standing there in the street waiting for them (awkward location with no sidewalk), I wished I had told them to just meet me in the Grandstand and given them their key there.