Here’s a couple of posts I made a while ago in another thread:
Unlike the Australian Labor example given by Giles, I don’t think there is any mandatory requirement to attend meetings of the local association. However, if you don’t show up very often, odds are you won’t be very influential in the local association, which is one of the reasons people join it.
As to who joins, usually the answer is the same as Giles’ answer: people who are generally committed to the party’s positions and want to see them elected, and are willing to donate time and money to achieve that goal. Some may join because they have ambitions to stand for election themselves some day, so they are earning chits by becoming a party stalwart.
Generally speaking, at a minimum these are the types of people who will cut a cheque for the party, or knock on doors, or stuff envelopes, etc., because they support the party’s platform and goals.