Hi, Claudia. We have an 'almost' only as she is 17 and her siblings are 23 and 26, both graduated from homeschool and college and are all but gone. Dd17 is different, tho, in that she is our most 'social' one. I never had to worry about finding friends or outlets for our older two. Their hobbies and sports interests were not group-oriented and they enjoyed their own company and quiet, individual projects. But it seems every parent gets one of those social-butterfly types... 
I wish I had more answers for you. Your dd sounds busy enough, her coursework coupled with her outside interests, lessons, meetings, etc. Is she wanting to do more things, or be with more people, do you think? You won't find young ladies her age available for friendships or anything else before 3:00 or whenever the local schools let out; and not to stir anything up, but they won't be as 'like-minded' as you or she might like. And like another poster said, we've not fit in with too many h/sing groups, either, and girls your daughter's age in a h/s group will be scarce as hen's teeth - most h/sing parents have put their older kids into a school setting by now. Good for you, holding out against the odds!
Here's a few things we've done, trying to keep our youngest occupied. She makes herself available for work just about anywhere, filling in for the church secretary or helping her when there's extra work to be done at church. She is paid for this, but would do it for free if asked. She also works at a local ice cream restaurant and goes to work after her schoolwork is finished most days, around noon or 1:00. They know they can count on her to be there and will even call her in for emergencies if someone's ill, etc. - they know she's home and can be depended upon. I know your dd is too young for a real job, but start putting the word out that she is available during the day to __________ (fill in the blank with babysitting, lesson-giving, dog watching, light housekeeping, reading to grandma, you get the idea...) You will also need to know your state's truancy laws before you let her out there much during school hours, of course.
Today, our Music Minister's wife called her to come help with the Christmas play scenery - the only youth, of course, that can come during school hours. She loves painting, etc. and yakking with all the church staff and people coming and going. She helps a handicapped friend of mine down the street by opening up cans for supper and putting pots on the stove, all the while laughing and visiting. She helped deliver Christmas baskets during the day with a church friend when no one else was free to help. And these are all not people her age, of course, but she sure can get along with most anyone and is very happy to have such a full schedule. An added bonus, she comes home tired and is content to stay home and be with family all evening
I've written a book and only wanted to give you a few ideas! Kick around some ideas with her and see if anything sounds interesting - I'd downplay the idea of only befriending or 'hanging out' with girls her own age. I wish that idea was more realistic, but it's one of the downsides of h/sing that has turned into an up-side for us - I hope it will for you, too...Liz