Doing a little Google for the Domain name...

Creation Date: 2007-01-20 07:03:00Z 

I'd say that's about when Roleplayer Guild became operational. One has to secure a domain before they can operate the site, so, unless RPG was hosted on a service like Invisionboards prior to this (or had a name other than Roleplayer Guild), early 2007 is the best guestimate.

Also I was on RPG in 2007, so... It has to be at least that old.