We moved in the last 6 months so that may be a factor.
This is huge. I moved in 3rd grade and it was the beginning of awkwardness for me. 3rd grade is also around the time when girls start to get really clique-y, and it's hard to break in and read social cues sometimes, especially if she is shy or has special needs.
What is your DH's input with your DD? Does she spend 1:1 time with him? Does he praise her and how? A father's attention is of course very important in building self-esteem.
I would:
Ask her if she wants to invite a few friends over or to a movie or playdate or something. Let her socialize on her own terms, and let her know that it's fun and great if she invites friends over or makes efforts to make friends.
Talk to DH and see what he thinks.
Talk to her teachers, she may be seeing behavior you're not. They also may have resources or ideas for you.
Also, don't let it show that you are somehow disappointed in her. My mom was very popular in high school and I was more intellectual and introverted. I could tell that she was sometimes frustrated that I wasn't more self-confident or outgoing sometimes growing up. DON'T let your daughter feel that YOU feel she has a problem.