Yes it is real. I know a lot of people may not believe it but it is real.
The problem with possession is if you let a spirit or entity into your mind to possess you, it may stay and not want to leave.
The spirit world is full of malicious spirits, demons, and evil entities. And a lot of these are searching for a home (that home would be inside a person). Sometime these spirits will pose as your dead relatives or someone dead they think you admire, in order to gain entrance to your soul. You just don't know what their intent is, and a lot of times these spirits have their own agendas, so you are taking a grave risk if you do this.
Once an entity has a hold of your mind it will start to influence your conscious thoughts, and you will find yourself battling these thoughts for control of your own mind on a day to day basis. Its not a very pleasant experience. The influence will depend on the type of entity or demon that goes in. You can do this, but you are playing with fire when you do. If an evil entity gets a hold of one of your friends over time you will see them begin to change for the worse.
But in effect just by using the ouija board you are in effect telling the spirits to posses you, since they have to in order for you to move the piece.