Do the individual pieces all have to be separate?
FollowMe, or the plugin Upright Extruder by Eneroth can automatically give the appearance of mitred corners, at least on one face. If you want three-way mitres where sides join, or where top and sides join, you will have to do them differently.
Otherwise, you will find a series of Skill Builder short videos (numbered 1-8) on learn.sketchup.com. One of them is for a frame with separate corner pieces, which is conceptually very close to what you want.
You will need to make mitred corner pieces, with the join edges to the uprights and cross pieces hidden.
Try that, and if you get stuck, ask again here.
PS. How far do you want to treat this as a learning exercise about Dynamic Components, and how much are you focussed just on getting your frames drawn? If the latter, and you don’t need dozens and dozens of them, it might be quicker just to learn how to draw them.
It would help if you can provide a link to the tubing component(s) you illustrate, on the 3D Warehouse. And also if you complete your profile. Which version of Windows? What is your Graphics Card - n isn’t a graphics card. Did you mean nVidia? If so, which model?