Some kids are going to be observers all their lives- and that's ok! I was introverted most of my childhood and liked to hang back and watch everybody else for a while before getting involved. Sometimes I didn't get in involved at all, sometimes I really just liked to watch better.
What kind of social behaviors are you or other main caregivers modeling? Trying some role playing might help. Also, your daughter might benefit from being reminded of one or two simple icebreaker questions, like "What are you playing? Can I play too?" or "My favorite thing to play is ------. What's yours?"
I wouldn't worry too much if your daughter develops one or two friendships in Kindergarten. I preferred to have one or 2 close friends each year during my school years (usually, one or both carried over to the next year). If you can, encourage your daughter to spend time with kids who live in your neighborhood because these friendships tend to last longer than the ones that they see exclusively in school. My best friend and I played all year long even when we weren't in the same class at school because we lived near each other.
If your daughter has absolutely NO friends, then I would ask her teachers to help her make a friend at school; sometimes they can consistently pair a child with the same other child for projects and a friendship emerges from that.
As your daughter gets older, she will need to learn to navigate our extroverted world in ways that work for her, but for now, she will be ok doing what comes naturally to her- sitting back and observing until she feels comfortable enough to join in.
I think the best thing you can do for your daughter is to accept her exactly as she is and not push her too hard to be extroverted. Just check in with her often to make sure she is happy with how many friends she has, and be ready with suggestions on how to make friends if she asks. (As an introvert myself, I am not going to be the one who can make those types of suggestions, but I bet you will get a lot of responses from other moms who know what to do!)
Best of luck to you and your little girl :)