My first STP was solo and I jumped onto as many pacelines as I could to start with. Like has been said, a lot of times people just tell you to hop on. Keep a bit of distance though, just in case. A couple of times I got on the end of a paceline where the other people were together and didn't want me in the middle of their line, so they would just signal for me to let them in instead of going behind me. Last year I rode with 3 other people and I didn't want us getting split up so I would signal to be let in after our group. It's funny how quickly those lines grow. I'd drop back after my pull and suddenly there would be 20 people behind us......
Whatever you do, watch out for the train track crossings. I saw people freak out and slam on the brakes to cross the tracks doing about 5mph. There were several near accidents.... and make sure your bottle cages will hold the bottles over some hard bumps. I could have picked up at least a hundred bottles last year....
Dean