It's not in your favour to do this. If you're smart you'd keep it to yourself and increase your chances of winning with less competition.
But please pass it on to someone that could be interested.
You know who I'm talking about. Maybe they've been posting screenshots of their Source Engine maps for 15 years and have never actually released their maps anywhere. Maybe they're always posting videos of impressive stuff they've made in Unity, but never actually release anything because it's not feasible to spend 20ms rendering clouds in any game.
I know it's really our job, and we're trying, but you stand a better chance than us. Maybe you could just shoot them a message and tell them about s&box. Let them know what we're trying to do. Let them know it's all free so they have nothing to lose. Let them know a lot of the skills they already have, like from GMod, CSS, Unity and C#, are transferrable. If they refuse, maybe get militant about it. Make everything about s&box. When they post a photo of their kid's birthday on Facebook, post about s&box on it. If they still refuse to enter the contest, unfriend and block.