While it can certainly be understandable that you might not want to restrict the number of people that would be able to get on your party bus and start enjoying it, once all has been said and is now out of the way there are a few practical considerations that might make it a bit more sensible for you to limit the attendance to only those that have received an actual invite. The reason behind this is that if you open up attendance a bit too much, you might find that certain individuals would start to attend who would be rather unseemly and might spoil the party bus experience dramatically for everyone else.
The key here is to make it so that only those that actually have an invitation can attend your party bus wine tour Napa Valley, and one way in which you can do this is to create some kind of code or password that people need to tell you before you can allow them on. Anyone can get their hands on a physical invite and there are lots of ways to fake these kinds of things too, so it just makes sense that you would use something that is a bit more technological.
Try to come up with a passcode or passphrase that no one else is ever going to be able to guess. Using a code but then making it something easy like a standard word or phrase would defeat the purpose of using this technique in the first place, so it is strongly recommended that you avoid doing this and try to think of a word that pertains to the party but is not immediately obvious.