Maybe I am naive, but, how about explicitly stating, by some high status member, that we would be very happy if they contributed here ?
Eliezer wrote the same thing about women. http://lesswrong.com/lw/ap/of_gender_and_rationality/ It was not exactly "Women, come, please" but it was clear they would be welcome to participate. It might have helped. Or maybe the increased percentage in the census result was due to something else ? How would I know...
And note that Eliezer did not forbid pick-up art discussion and whatever You guys hold dear.
I could try and write a similar post as was that about women, but I am a small fish in this pond.
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.