How do you become a member of a caucus?

Answer 1

In general, local party caucuses are open to all members of that party in the local region. In most US states, there are party caucuses that elect local party officers. If you are asking about the Iowa caucuses, Iowa has same-day voter registration, so any eligible voter may show up at a caucus, register (or change their registration) to that caucus's party, and participate in that caucus.