I have evidence-based insight into the gender ratio issue. Although it is not a solution, I think it will help everyone understand the problem better.

I have some unpleasant news which is related to IQ (and LessWrong has a higher than average IQ according to the member surveys). There's something up with genetics and intelligence that goes like this (references included): Although men's and women's IQs are the same on average, there are far more gifted men than women. The explanation is that high intelligence is due to a mutation. Men are more affected by mutations. Therefore they are about twice as likely to get both beneficial and detrimental intelligence mutations, which is why they are unbalanced for gifted populations but even out in population averages. "Diseases inherited in an X-linked recessive pattern mostly affect males, because a second X chromosome usually protects females from showing symptoms." (From: How Are Genetic Disorders Inherited?) See also: Mensa's demographics page where they report a 33% female : 66% male ratio (for the top 2% in IQ).

I've heard it reported by people with very high IQs that the higher the IQ range, the worse the gap is. This may be true if higher IQs require multiple mutations.

If refining rationality, science, etc. or the specific forms of these that interest LW members tend to appeal most to people with high IQs, this ratio is probably, unfortunately, going to affect groups like these whatever you do. Of course, if the male to female ratio is 2:1 in Mensa (unsure what their average IQ is, just that the minimum is the top 2%) this means there's probably room for improvement. However, short of genetic engineering or brain implants, the gender ratio problem is likely to persist for high IQ groups like this even if a perfect strategy was used for making women feel more comfortable.

If the important thing is to be able to have children one day, then creating or using a service for finding intelligent women who are willing to have children for others would be one way.

Another idea might be encouraging women to work extra hard to refine their rational thinking skills, explaining that they'll interest more intelligent men that way. As we know, women can be picky and they tend to have a preference for intelligence. If they know that smart guys are looking for rational women, some of them may devote attention to it the way that many women currently devote attention to hair and make-up. Not all women will be interested, and it won't solve the incompatibilities that IQ differences cause, but it may help bridge gaps that aren't too great and lead to there being more female rationalists.

I feel that addressing the "creepiness" issues could go a long way to get interested women interacting more. Nerdy men are definitely my type but I'm often put off by their behavior. I'd like to date more, but the behaviors I describe here are disheartening, so I'm not nearly as motivated as I could be. Understanding women better is a great first step. I'm glad to see that the guys are looking into what's going on.

Unfortunately, I can't provide you advice or evidence and don't know that anybody can - I'm not aware of any projects or experiments to specifically try to increase the number of females in a male dominated social group.