How to bind to method's result in Telerik RadGridView with ObjectDataProvider?

Question

I have Telerik RadGridView. I bind IEnumerable collection where UserRole is my class which describes user's role and contains collection of permission. Imagine that collection of permissions is an array.

XAML where I bind data looks like this:

<telerik:RadGridView x:Name="grdRoles" Margin="5"
                                 IsReadOnly="True" BorderBrush="#FF595959" BorderThickness="1" 
                                 ItemsSource="{Binding}"
                                 AutoGenerateColumns="False" >
        <telerik:RadGridView.Columns>
            <telerik:GridViewDataColumn Header="Role name" DataMemberBinding="{Binding RoleName}" MinWidth="150" />
            <telerik:GridViewDataColumn Header="Permissions" DataMemberBinding="{Binding Source={StaticResource something}}" MinWidth="300">
            </telerik:GridViewDataColumn>
        </telerik:RadGridView.Columns>
    </telerik:RadGridView>

I need to convert an array with permissions to string that looks like "Perm1, Perm2, etc.." I created

    <LocalControls:SettingsPage.Resources>
    <ObjectDataProvider ObjectType="{x:Type LocalControls:Class1}"
                    MethodName="GetPermissionsString" x:Key="something">
    </ObjectDataProvider>
</LocalControls:SettingsPage.Resources>

with class

    public class Class1 {
    public string GetPermissionsString() {

        return "aaa";

    }
}

According to this string

<telerik:GridViewDataColumn Header="Permissions" DataMemberBinding="{Binding Source={StaticResource something}}" MinWidth="300">

aaa is shown in Permissions column for each record in grid.

I need to pass each UserRole object to GetPermissionsString() method to convert an array to string.

The question is that I don't know how to do this. Can you give me some example?