They put feelers out to celebs to attend an event and RSVPs are given so that if they can/want/do attend, the tickets will be ready once they arrive. I've been to several premieres that Josh was a part of and they basically go up to a talent section where there are alphabet 'stations' for talent/actors/etc. to go and get their tickets to see the premiere if they don't do the red carpet. But if they do, then they just walk on in since their PR people take care of all that beforehand.

To see red carpet events, it's really easy if you live in the area. This one is at the Arclight in Hollywood and I've seen Josh do a couple premieres there. If you show up early enough, you can be along the red rope and watch them arrive, pose for photos & a lot of times, they come over to the fans for autographs & a photo or two.