The psoas, which are actually called "iliopsoas" as they include the ilacus + psoas, lie deep beneath your abdominal muscles and intestines.

They attach to the sides of your lumbar vertebrae (low back spine), starting at your last (12th) rib and continuing down to the pelvis.

Psoas combines with iliacus in the groin and follows down into the top of the femur. If you imagine that your hip bone on the front of your pelvis is a bowl, you can envision how the muscles also attach to the underside of the bowl (I like to think of it as how you would grease up a bowl-the grease would stick to the sides of the bowl. That's what iliacus does, in essence).

Trigger points in the psoas muscles can contribute to both low back pain and the groin. 

Causes are often:
✔️sitting
✔️overuses (athletes)
✔️physical and emotional trauma
✔️may be caused by tension or fear

Here's a video I did to show you how to help loosen them up.

All you need is a foam roller. 

I can also work them during your deep tissue barefoot massage. I will say that it tends to be unpleasant, but if that's the source of low back pain, you'll feel better right away.