LINQ GroupBy and then OrderBy a property from the previous query

So I have a CategoryAttribute class:

[AttributeUsage(AttributeTargets.Field | AttributeTargets.Method, AllowMultiple = false)]
public class CategoryAttribute : PropertyAttribute
{
    public readonly string name;
    public readonly float order;

    public CategoryAttribute(string name, float order)
    {
        this.name = name;
        this.order = order;
    }
}

That way I can categorize my class members to draw in the inspector (Unity3D) in groups, like so:

Now these are the default groups ("Fields" with an order of 0, "Properties" with an order of 1 and "Methods" with an order of 2).

Now I want to let users to create their own categories with their own names and orders, so say I wanted to create a "Debug" category:

[Category("Debug", 3)]
public bool debugMode;
[Category("Debug", 3)]
public bool drawGizmos;
[Category("Debug", 3)]
public Color gizmosColor;

Now both these three fields will be drawn in a category called "Debug" after "Methods" (cause it has a higher order) (I know the syntax is repetitive, I'll do something about it later...)

Now I can't seem to group my members by the category name, and order those groups by that category order number, I tried but bumped:

var members = fields.Concat<MemberInfo>(properties).Concat(methods);

var userCategories = from m in members
                     let cat = m.GetCustomAttributes(typeof(CategoryAttribute), false)[0]
                     where attr != null
                     let cat = attr as CategoryAttribute
                     group m by cat.name into g
                     orderby cat.order // bump!

And:

var userCategories = members.Select(m => new {m, cat = m.GetCustomAttributes(typeof(CategoryAttribute), false)[0] as CategoryAttribute})
                            .Where(x => x.cat != null)
                            .GroupBy(x => x.cat.name)
                            .OrderBy(x => x.cat.order) // duh!

Now I can fiddle around with projections and eventually figure it out, but it might not be the best so I thought maybe someone could help me do this the right way.

Again, I want to group my members by the category name, and order the groups by the category number.

Thank you!