Hi - I'm new to Pathfinder, and I saw that a Beginner's Box game had been started for PbP a couple of months ago. I am interested in joining one if a more experienced player can guide me/us through the PbP methodology and of course GM us in the Beginner's Box dungeon or suitable alternative.
To learn the PbP methodology, read. Read a PbP campaign's recruitment thread and campaign info thread. Then read the gameplay and discussion threads until you feel you get the idea. Then do the same with another PbP campaign, and another, until you feel comfortable with how it works. That will guide you. And then if you have specific questions, you could post those somewhere such as the "Online Campaigns General Discussion" forum.
To join a PbP, watch the Recruitment subforum. And I mean watch it like a hawk, because those things fill up quickly! Many players want to join these games, and not as many GMs can manage the work and commitment that goes into running one.
And if you want something more specialized such as a Beginner's Box game, that will be even harder. If, after watching that Recruitment subforum for several months, you don't get any opportunities, you may have to consider starting and GMing one yourself.
In general, PBP revolves around a recruitment thread (1st), a discussion thread (outside game) and a Game thread (in character play).
Once the GM has determined who is playing and sometimes what their characters are in the the recruitment thread, they'll close recruitment and start the game.
The GM usually specifies, or develops between himself/herself and the players, a posting "requirement" describing how often people are expected to post, what happens if you can't post, etc.
In the game that I'm currently running, called After Dark, we've kept adventures at about 1 - 2 months real time, and I generally level the PCs after each adventure. So we're moving at a decent pace. It's pretty much up to the GM how he/she runs XP/leveling, how/if they deal with mapping, what sort of house rules, etc.
You might have trouble if you insist on BB, but there are plenty of GMs starting new games often, and most of the ones I've seen are pretty friendly to and understanding of new players.
Best of luck.