How can you see the ghosts or invisible people in the shaded woods

They are not invisible, just very hard to see when the camera is moving. Move slow, don't move the camera too much and look for the edges of the figure. Once you find one, move straight towards them slowly, and look for them to notice you. They roam around a little in short patrol lines.

Stop moving and rotate your view as best as you can to keep them in front, since a backstab is probably going to be a one shot kill. When they get close enough to commit to combat, attack. When they take damage, their health bar appears briefly.

It is difficult to judge because the closer they are, the more your player model blocks them from view.

If you have to retreat, either turn and sprint, or backpedal, trying to move your view slowly enough that you can still track them. Try to be mindful enough to retreat along the path you approached to avoid attracting the attention of a second or third one.

Do not open any chests etc. before you have taken care of the one(s) nearest the object.

The worst part about this area is that the other players' ghostly apparitions are easily mistaken for enemies. The enemies are less grey.

If you're playing the Scholars of the First Sin edition, you've got an option that gives you a better chance against these enemies. Obtaining the "Eye of the Priestess" item from Frozen Eleum Loyce will make all ghost enemies in the Shaded Woods completely visible and capable of being locked-on to.