I have tried to replicate you problem with float grouping, but everything works fine on my end. I'm sending you my project so that you can verify it locally on your PC.
The problems with designer is that we generate a set of sample data - list of Customers. In this case when you add a new group descriptor (in design time) its member property should be equal to a property of the Customer type. However If you bind your grid declaratively (via ObjectDataProvider) Visual Studio will correctly bind the ItemsSource property and everything will work as expected. I have done just this in the sample project I'm sending you.