Let's say your subforum is about Dogs. If you hit the Community button in the top menu, and then enter "Dogs" in the search by interest box, you'll see this:
http://www.gaiaonline.com/profile/index.php?view=search.ShowSearch&type=interests&item=dogs&x=15&y=10 You might want to invite those people to the guild, and specifically mention your subforum. The feature of searching by interest can be a very helpful way to look for people who might be interested in your topic. Brainstorm some other words... related like: AKC, chihuahua, obedience. Now there's is no way of knowing whether these people are online or not, so don't expect fast results, but if you invite 100 people, generally within a week they will have all looked at the invitation. Perhaps 30 will join.