Well, the official rule is only one vehicle per campsite. So, if you have a vehicle at your site, you could park it in the parking lot up at the Reception Outpost, and then your guests could park at your site.
Or, your guests could park at the Reception Outpost, and ride the bus back to your site.
Or, you can break the official rule, like many people do, since it is no longer enforced, and your guests can squeeze their car in somewhere at your site. It is unlikely that anyone will say anything to you about this.
When your guests arrive at the guard shack, they will need to show a photo ID, and tell the guard that they are here to visit a registered guest. The guard will probably wave them on through without further question, and they will then have access to the Fort, including the ability to drive to your site.
TCD