I like the idea of a team sport or other team activity. It's not so easy to tell at this age, but I am a person who avoids groups of people, especially if I don't know them well. It's hard for me because I am very sensitive to other people's thoughts and feelings, and it took a long time for me to get a handle on that. I didn't even realize what it was at that age, but now I can tell that it is information overload, with people and "stuff". I can get overwhelmed in the mall and at concerts.
I'm not saying that this is your daughter's issue. I'm just saying that while she certainly needs to learn how to relate to people, even in group settings, maybe you could start with teaching her how to be content (even happy) on her own. That foundation will give her more confidence to relate in groups. If she shines as a kid who doesn't mind playing alone....