This was totally my kid, and two things have helped her come out this year (3rd grade): therapy and Girl Scouts. I grew up being painfully shy and I knew what it was like to want to get involved with friends but just feel paralyzed and not know how to just get in there. So when we started seeing signs of the same with DD, we spoke to her pediatrician and got her into some therapy. I was hesitant because I thought of therapy as something you go to for a serious issue, but our ped helped us understand that for DD, it IS a serious issue, and better to get a professional to give her the tools to help herself now, rather than waiting for her "grow out of it."
Also, her Girl Scout troop is amazing and the GS ideals really helped her be more assertive.