There are plenty of women who ride and race cross country (I'm assuming that's the kind of riding you're doing as opposed to 4X, downhill or trials). Turn up to a race and you can meet them.
Most of us ride with whatever company we can find and keep up with, be it male or female. I'd suggest ride with whoever has rostered days off that suit yours regardless of their gender. A good "teacher" is going to help your riding and in my experience some of the best are men.
If you want it to be an all female thing, then as smc said there are some specific girls only mtb rides. I'm about to post the next one in the plan a ride section (can't bring myself to put it in here, as I've never quite understood the point of this subforum).
The FTF ride on Wed night usually has a "C" group which is at least one guy and one or two women. Occasinally more. They're inclined to be patient and helpful if newcomers go along and join in.
There's always an A group if you're quick enough (and they won't be so patient if you're not!), and sometimes a B group. There are women who turn up and ride with the faster bunches (occasionally). Please have an mtba licence or FTF club membership if you do this ride for insurance purposes.
Give me a bit of time and I'll be back out there willing to ride with anyone (pending babysitting arrangements). Gonna take a while to regain any form!