My son is 10 years old and has the same problem. We tried everything and nothing really worked until we went on medication. It seemes to be a hormone difficency that he will grow out of but in the meantime he was embarrassed at sleep overs and we had back up plans incase he had an accident. I don't know how long we will be on the meds, but in the meantime, it has helped him.
Approximately 5-7% or so of children her age are enuretic at night. You really needn't do anything about it if you wish. The situation will eventually improve. There are interventions (prescription medication, wet/stop alarms) that can be employed, and you might want to talk this over with your daughter's pediatrician. Rememebr: the interventions are entirely optional; there is no compelling reason to intervene.