use a custom skin with a rect as background
or use a bordercontainer with a horizontal or vertical layout
<s:Rect width="100%" height="1">
But if you need this often, creating a skin like kokorito said is best practice in my oppinion.
Also, you can consider using the SkinnableContainer which has backgroundColor (etc) properties, but has more overhead then Group.
If it's a desktop App, you won't probably see any difference.