Yes, I'm female. No, I can't do 15 pull-ups, I can only do them with the big elastic band, which is what I meant by "medium band".

And we didn't just do 15, we did 5 rounds x 15 =
75 pull-ups!!! (But I did them with the help of the elastic band!)
Oh, and we do kipping pull-ups, not strict pull-ups. They are much easier than strict pull-ups. Kipping uses momentum to help you get up. It takes practice to get the rhythm, but once your body gets it, it is much easier.
They have these big elastic bands you can use, which help a lot. You knot it to the bar, and then put one foot in the elastic band, so it helps lift your weight for you.
When I first started CrossFit, I couldn't even do them with the biggest, fattest elastic band they had available! I was doing jumping pull-ups, and not improving at all, which I found really frustrating. So I asked a trainer for advice, and she suggested lowering the rings and positioning myself at an angle so that my heels were touching the ground, but my body was as close to horizontal as possible, then lift myself so that my chest aligned with the rings, then lower myself again. Sorta like reverse pushups, pulling myself towards the rings and lowering myself again.
I did those ring-pulls after every workout for a couple of weeks, and was finally able to do pull-ups with the fat band, yay! The first time I did pull-ups with the band, the muscles at the bottom of my shoulders were super sore, so I guess that was what was limiting me, and what the rings helped me build. I used the fat band for a few weeks, then a slightly less fat band for a few weeks, and then the medium band.
I'm still at the medium band and will be for awhile, because the next band down from it is way thinner. My old gym had a lot more variety in band sizes than this one does. For some reason, the owner of this gym thinks jumping pull-ups are better, but I sure as hell get a lot more out of band pull-ups than I do out of jumping pull-ups. (I find band pull-ups much more difficult, so I get a better workout from them and imagine I'm building more strength that way.)