"Only" children are often more comfortable interacting with adults than with other children simply b/c that is what they have experienced. It sounds like she just needs some practice!
There are lots of ways to do this... do any of your friends have children around her age? If so, invite them over for a play date. Have a structured activity for the children to do as an "ice breaker"- craft, baking, etc. This will allow you to facilitate the interaction and help your daughter "play" with her peers. Then give them some free time and see what happens.
You may also want to get her involved in an outside activity like dance or gymnastics. There is a really good chance that your daughter's language skills are strong and that it is difficult for her peers to understand her. It is also likely that she is used to playing what she wants to play (not in a bad way, just by default) By enrolling her in an activity where they are all learning the skill, you are allowing her to be "on the same level" as the other kids.
You could also take her to the park on weekends- running around and swinging is a great way to meet friends. Giving her some practice with how to communicate with her peers will make it easier for her to negotiate the social world.