Shadow people are very real. Similar to orbs, they are a natural occurrence that are everywhere.
If your mind is open enough, you will see them as they are very low-vibrational astral entities.
Keep in mind that the lower vibration that something has, the easier it is to manifest and be seen on the physical plane.
In general, shadow people are very harmless deceased souls that are just passing through. While others are attracted to negative energies such as people who are constantly depressed, use drugs, and inhabit places where there is a lot of negative energy (murder scenes, mental asylums, hospitals, cemeteries, places where there is domestic violence, drug use, sadness, etc).
It is easy to change a negatively-charged placed into a positive one by playing upbeat music and by being positive, and active in the area. Any negative presences would move out as they are no longer compatible with the higher vibration of energy being introduced.
HS.