Why does this girl invite me to several group hangouts but barely talks to me alone?

A girl invites me to hangout with a group of classmates a few times. Arcade, eat out, party at a house, etc. When it ends up where it's just the two of us standing next to each other, she is quiet until someone from the group joins us again. Then, she becomes talkative talking to everyone, including me.

She is only talkative when we are both in the group. But if it's just the two of us alone, she is quiet.
Why does this girl invite me to several group hangouts but barely talks to me alone?
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