No there's really no danger to be hypnotized.
Being hypnotized only works on people that are willing to be hypnotized and a person can be easily taken out of hypnosis.
However if a person has a severe mental illness then hypnosis may not be safe for those people but if you're in good health and have no mental health issues and are a willing hypnosis subject it's safe.
The hypnosis is best done by trained and professional hypnotist but even others may be able to safely hypnotize some people.
Also you cannot be hypnotized without your knowledge because you have to know about it and be willing or the hypnosis will not work.