If I have a sinus infection am I contagious to others?

No. A sinus infection is a localized infection of the sinus cavities/passages. It is bacterial. There is usually no way to spread this to another person. Nothing is absolute but in generally my answer is no.
Sinus Contagious? Usually sinus infections follow viral upper respiratory infections. While the virus is contagious, the bacterial infection that is part of sinus infections is not contagious.