Friend asked me to commit to a group I don't want to. What should I do?

My best friend wants me to join the Honor Guard for our high school's marching band. This is a group where we carry the American flag and other flags before the band preforms. He is the only person interested in doing this. There is no gain to joining. I would have to practice a few hours a week only to be labeled as a major geek. I lied and said that I am interested but don't know if I can due to a busy after-school schedule. I care for my friend but I honestly don't want to do this. Should I come up with an excuse (such as not being available after school) or just straightforward tell him?