If a 15 year old wants to watch IT does it need a company of legal age?

Yes

The movie "It" is rated R because it contains violence/horror, bloody images, and for language. The fifteen year old needs to go with an adult and or legal guardian who is 18 years of age or older. Otherwise, he or she will not be able to see the movie by him/herself.

Yes the people need a adult with them to watch the movie because it is rated R. It is a horror movie.