The shoulders are a common area of habitual tension due to the nature of the startle reflex and associated stress states of the nervous system. The shoulder girdle is a very mobile structure and not well suited to weight bearing. Although the joint that connects it to the torso via the clavicle is at the front top of the rib cage (top of the sternum), the shoulder blade (scapula) itself hangs down the back of the ribcage off the other end of the clavicle. As the arms connect to the shoulder blades, this means that your arms are skeletally part of your back, and it's helpful to think of your arms as being part of your back.
In fact, the latissimus dorsi muscle is a large sheet of muscle that connects from the upper arm to the spine and all the way down to the top of the pelvis. It's useful to think of the full length of your arm, then, as starting from the top your pelvis. Your arms are really embedded into most of your torso, the only part of your torso that doesn't have arm muscles is your abdomen. The width of your arms is from the centre, at the front from your sternum, and from your spine at the back. When you use your arms, you use your torso, but it's the back that tends to become most neglected in coordination. Balance and poise in the torso allows for more effective use of your arms.
If you make a tight fist you'll feel the muscles in your forearm tighten. They're finger muscles! Want to move your forearm from the elbow? That's done by the biceps and triceps of the upper arm. Want move your upper arm? Keep back tracking, the muscles of the torso. When you use your fingers to play guitar, they're supported by your torso, and especially your back.
The two main tension issues with the shoulders is pulling them up towards your ears, and pulling them forward, narrowing the chest. The shoulder and neck muscles are interrelated, the trapezius muscle (covering the upper back) is both a neck and shoulder muscle. As much as you need to release the shoulders down, you also need to release the neck into length with the classic AT "head forward and up", which I recommend you do first. The weight of the arms attached to the shoulder blades is enough to allow the shoulders to hang down the back. There is a point at which you need to raise the shoulders functionally to reach above your head, but that's not a requirement for playing the guitar.
With regards to bringing the shoulders forward and narrowing the chest what you don't need to do is pull and hold them back. This has to be one of the most repeated pieces of advice with regards to posture, and one of my biggest bugbears. Essentially it's unhelpful and doesn't work, and you'll know it doesn't if you've ever tried it, it's unsustainable. Seriously, why go to all that physical effort to solve a problem of too much physical effort? By releasing the chest into width, the shoulders will naturally hang down the back.
In terms of body language, an aspect of pulling the shoulders forward and narrowing the chest is an associated emotional state, especially that of vulnerability. Recognise that. It's totally OK to feel vulnerable, but you don't have to broadcast it physically, and by not displaying it physically others wont respond to it negatively so readily. I personally think that being open physically but still present to your vulnerability is a powerful artistic communication in it's own right. Your stage presence will certainly benefit from this if you're generally a shy performer.
Anatomy and mechanics aside, Inhibition and the quality of your nervous system is the starting point to avoid excess tension in the shoulders. Your shoulders, like the head/neck relationship, can be another useful barometer as to what's going on with you, as you won't simply have tight shoulders, but be tight throughout.