The question of allowing transgenders to join a youth group for one specific gender is pretty much one of social evolution.
Regardless of what may have constituted "the norm" in times gone by, the times, as Mr Dylan once said, they are a -changin'. Society doesn't stand still. It adjusts. It adapts, it alters. That's all that's happening here. And in my opinion, that's a good thing.
If society is becoming more inclusive, and that message is being passed on to young people by positive action then I say "Good-oh".
I was once a Brownie. I enjoyed it. It was fun and there was a feeling of belonging. Being a Brownie was to embrace girlhood as something special. I never moved up to the Girl Guides (the equivalent of your Girl Scouts) because I was involved in my ballet classes and being a member of St John's Ambulance Brigade. But had I done so, I think I would have been happy to welcome transgenders.
A group is, by it's very nature, intended to be inclusive. That's the whole point of having a "group"..!! Excluding transgenders only serves to support exclusion and discrimination against people who - I believe - have a valid claim to recognition of their true gender.
If society is to evolve in, what I believe to be, the right way then inclusion and acceptance must be a part of that. Not only will it be beneficial to the (for want of a better word) trannies themselves, but it will also help to familiarise girls and young women with those who were born in the wrong body, at an age when adult opinions and attitudes are starting to form.
Accepting transgenders into the Girl Scouts is a big step down the road to taking society where we want it to go.