The utilitarist is that who adheres to utilitarism (the theory that cites the best thing to do is to sacrifice "the happiness of one to the advantage of the majority").
Everyone is an utilitarist in some way. I am being utilitarist now by writing this text in the example box to exemplify it. In this (example) case, is more of a (dub) case of "hedonist utilitarianism".
The philosophical theory that has as the golden rule : "The happiness of one is less important than the happiness of the majority".
In this author's point of view (and Audous Huxley for example), the cited "majority" in the utilitarism, is the most number of people that are benefited from an action: Jimmy has a cake with 8 pieces, but there are 7 hungry people around Jimmy, so Jimmy is being an utilitarist if he is going to share the 7 pieces of cake with other people out of his own will.