Several of the groups I go out with have a good balance of male and female riders or even more women, against the general trend. The main differences from other groups are regular cafe/pub stops, willingness to stop and investigate interesting things on the way and a non-competative pace.
I think that's probably the key to inclusivity.
I'm speaking from a position of ignorance, of course, as I'm hardly traditional cycling club material myself, but it strikes me that the women who want to ride fast (if not actually competitively) will have no problem actively seeing out others and eventually be thwarted by the general lack of numbers rather than attitudes or policies.
Meanwhile, more leisurely (which doesn't necessarily mean short) group rides will attract all kinds of riders, and are inherently women-friendly. The Friday Night Ride to the Coast deserves honourable mention, I think, as it does a particularly good job of it. I'm not sure whether that's because it appeals to us weirdos, or because riding at night is a good way to avoid traffic without being limited to closed roads and cycle paths, or because overnight rides make things like childcare a lot easier to organise. Probably all of these things.
And then there's groups like the astoundingly successful Stourbug. They certainly seem to have the right idea.
And let's not forget YACF and its ilk. You guys have somehow managed to convinced *barakta* to buy a Trice. How much encouraging-women-to-cycle do you want?