How to fade a Union within a group?

Because your model consists of separate unions, it would be best to iterate through all the parts and fade them each using TweenService. You can use the function GetDescendants to get all instances of a group. Then, check to see if the instance is a part. Then you initialize a tween on the instance. In order to initialize a tween, you must specify an instance to perform on, a TweenInfo instance, and a table containing all the properties to be tweened. All of this would look something like this:

local TweenService = game:GetService("TweenService")

function fade(model , transparency , fadeTime)
    local descendants = model:GetDescendants()
    for i , v in pairs(descendants) do --iterate through descendants
        if v:IsA("BasePart") then
            local info = TweenInfo.new(fadeTime)
            local goal = {Transparency = transparency}
            local tween = TweenService:Create(v , info , goal)
            tween:Play() --start the tween
        end
    end
end

Read more about TweenService here:

http://wiki.roblox.com/index.php?title=API:Class/TweenService