If you’ve been trying to get anyone and everyone to go see your friend’s band perform, then you could call yourself a promoter of that band. A promoter is a supporter, advocate, or booster for a person, group, or event.
A promoter is an active supporter of something. A promoter of democracy, for example, is someone who tries to spread democracy throughout the world. Being a promoter is also a profession; one might be hired to make arrangements and generate publicity for some form of public entertainment, like a boxing match or a concert. Perhaps if you tell enough people about your friend's band, the band will hire you as its official promoter.