I used frozen white grape and rasberry concentrate. One can is enough for my taste, sometimes two. ( in a five galeon batch ). Remember to let whatever you use to sweeten, set for at least a day before adding more. I can't do unfermented cane suger, and really like the flavor profile of juice concentrates better anyway.
If you already have a strong rasberry flavor you can use apple juice concentrate instead. It is more mild in flavor.