It's the adults who have the problem. The younger they are introduced to each other, the better. Help the adult in charge by giving information that would be helpful. Stay close at first to help the others understand the best ways to interact with your child in an activity. Then slowly back away to encourage others to step in.
Kids love helping kids.
Talk to the other children, making friends with them yourself. Talk about how your daughter is more like them than different. Tell them she learns things at a slower pace. The National Down Syndrome Congress is a wonderful resource. I always wonder the same thing for her.