There's a site guide on it, but it was way in the back of the Site Guides section, so I don't blame you for not finding it.
Anyway, the existing Founding Members were given that status because they joined the forum early on and contributed either by posting or by helping out with bug fixes (
@Octavia and
@Rem were extremely helpful with this). It's not something anyone can or ever has paid for.
You can become a Platinum Member eventually, but I need to work out if the Platinum Members want a nomination process for it and how they want it to work.
I just know someone is going to think you're being serious and I'm going to get another message from someone trying to bribe me again...