Grouping is easy:
var grouped = ac.GroupBy(a => a.Cusip);
But you want only those groups with more than one member?
Try this:
List<AccruedClass> grouped = ac.GroupBy(a => a.Cusip).Where(g => g.Count() > 1).SelectMany(g => g.ToList()).ToList();
[edit]Forgot to encode HTML tag characters...[/edit]