We have a Premier Camping Pass for the Eric Church concert. As it turns out the couple staying with us will be arriving a day later. Is there a way to get them in without having to purchase another camping pass?

No, there is not. You'll have to buy another camping pass.

If they aren't in the same vehicle as you they will need there own camping pass. Also if they arrive a day later than you they won't be camped next to you, you have to arrive at the same time if you want to camp next to each other

I think you pay per vehicle so if they come with a separate vehicle, they'd need their own pass