If you’re a big fan and promoter of something or someone, you're a booster. Go team! A booster provides support, like a kid’s booster seat, a follow-up dose of medicine, or a booster rocket that launches the Space Shuttle.
Boosters of teams, groups, or organizations often form official clubs to coordinate their fundraising and event planning. Another kind of booster is an engine or rocket that's the first stage of launching a space vehicle into orbit. And in medicine, a booster is a second (or subsequent) dose of a vaccine that helps the initial dose work better. All of these boosters assist, encourage, or boost in some way.