Just based on personal experience, I think I gained the most improvement when I rode with women who were right near the limit of my skill level. I think this is best when you get past the initial introduction to the trails and have some confidence (still a beginner, but capable of doing some of the climbs and most of the descents). Everyone recommends you ride with people who are better than you, which is good, but they can get a bit ahead and you really won't gain that much. When I rode with a group of women who were right at my max (maybe a tiny bit better than me) it really pushed me. I pushed myself harder riding with them because I knew I could keep up and seeing other women doing climbs or descents I had trouble with pushed me into thinking I could do it, and I did. I rode better with the women than I normally do with my husband because I know how matter how hard I push, I can't keep up!