I went alone to my first ProgPower, which was ProgPower III. I knew no one in person, and only a couple of people electronically. By the time I left, I had several new best friends, and a large group of people to look forward to seeing the year after. And the year after that. I haven't missed one since, but unfortunately I'll be breaking that streak this year. Regardless, I echo several previous posts that advise you to just go up to people and say hi, whether in the venue or your hotel. This is one of the friendliest crowds I know of.
Ken