http://incubator.duolingo.com/apply
I firmly believe that Interlingua's biggest barrier to popularity right now is the absence of good introductory learning materials (especially for English speakers). There are the tutorials on the website and there are hard copy books that you can buy, but this doesn't cut it in the digital age. Esperanto has had a fantastic teaching site at http://en.lernu.net/ for years, which certainly contributed to the online Esperanto renaissance.
Furthermore, there is an active Duolingo incubator project for Esperanto already: http://incubator.duolingo.com/ and there are less active ones even for fantasy languages like Klingon, Dothraki, and Sindarin Elvish. So I'm certain Duolingo will accept an Interlingua project despite being less popular than Esperanto.
To some degree, preference in auxiliary languages is subjective, however I think Interlingua stands above the competition because of it's broad intelligibility at first sight. I think that this actually gives it an advantage when justifying to people why they should be learning Interlingua versus another auxlang. Because of this, I believe Interlingua would be a lot more popular if there were better freely available teaching materials.
Much as I would love to help, my Interlingua skills are, unfortunately, still lacking to really contribute in any meaningful way to such a project.
So what do you think? Any volunteers?