It depends on the state you are in, since there are schools which do not allow homeschoolers to join extra curriculum activities, especially a band. There are schools which give guidelines on which activities homeschoolers should join, so it would be best to check with the school.
Check with your local schools to see what activities homeschoolers can join. In my area, homeschoolers are very active in school clubs (including bands), sports, and other activities. It seems to vary widely by state and city/location. Good luck!