There's a new Facebook group of women that mountain bike together called Singletrack Sistahs.

First ride at Theo is April 9 at 4p!
First ride at Hillside is April 10 at 6!

2012 all-girl group rides:
(beginning this spring)
Mon's 3:30-4-ish at Theo
Tues's 6 pm at Hillside
Wed's 6 p.m. at Elm Creek
Thurs's 9:30 am at Elm Creek

Sue described the group for me:

We're a mountain biking group for chicks of all kinds who either ride dirt or want to learn how. We're starting some group rides this spring anyone can jump in on. Mountain biking is pretty different from regular riding in terms of technique etc, and having the right bike makes it much easier/more comfortable too. If anyone wants to join our group you are very welcome, and if anyone has any questions about getting started mountain biking I'd be happy to answer them. You can message me. :) See you out there hopefully!