Well, perhaps you're not aware, so I'll fill you in on a little secret.
1. Forums and BB software are 10 a penny. The market is flooded with them and they're all more or less identical
2. People will join if they have something to gain, if they have nothing to gain, they won't join. Regarding things to gain, see point 1.
3. PHP Programmers (at least the ones worth their salt) won't program for free. It seems that this software is being purpose built for yourselves, which suggests you're looking for free programmers. I won't say they don't exist, but good luck on finding them.
4. Working as a team is nicer, when the team are physically present to verbally communicate ideas, and they're all working at the same time. I know from experience working with a team over the net is very difficult. It's not impossible, but very difficult.
5. There are more, but I think 5 is enough, so I'll make this the crucial one. A PHP based forum already exists, it is created through OOP, it is free, it is used by millions, it is extendable and open source, and it's the one which is running these forums - phpBB! I personally can't think of a single reason why anyone would want to sit down and make another one, when a perfectly good one already exists. But if you care to look, be my guest.