While I don't want to run an unmanageably large party, we're always taking standby players to join in when there's room. If you want to get in on this campaign, please:
1. Contact me first, preferably on Twitter.
2. Have a Twitter, MetaFilter, or similar account, with a history, somewhere.
3. Submit a 1st edition AD&D player character sheet, rolled to the appropriate level (I'll let you know what level the party is currently at when you ask to join).
Standby players will be added to the game, when possible, in the order in which I receive their character sheets.
Update: if you aren't familiar with 1st edition AD&D, or you don't have the books, the free-to-download OSRIC system appears to be fully compatible with the rules we're using.
Another update: all new players joining will start as members of the Adventurer's Guild in Guffin. This is to provide an easy and logical in-game reason to integrate them into the party. All new players will start at the level of the lowest-level member of the existing party, and will begin with one or more magical items at the DM's discretion. See this post for a little more about the Guild.