You can work in partners with bands if there is nowhere to anchor the bands.
You can start by teaching them about the stable shoulder position and how to replicate that.
The more they understand about the shoulder and how it works the more they will begin to understand about movement of the shoulder and what they are looking to improve.

You can start with basic elements using PVC. These are pretty inexpensive.
Before addressing shoulder position you have to address the shoulder’s carriage or chassis to make sure it is organized. If its not organized you’ll never get to the bottom the cause.
You can start with the inside of the neck, lats,  and first rib mobs.

You can make PVC rollers and cut a yoga map to cover then if you want something to cover the PVC.
Message me and I can send you directions.
Episode 341: Runner’s Shoulders

Episode 89: Bro, Your Internal Rotation is Lacking

Bilateral Shoulder Flexion