The meaning of "registered" member of a party is different from state to state, Giok.

We have "primaries" here, which are basically means to narrow down the field of candidates. For example, there may be 4 people who consider themselves Democrats and want to run for a particular office, such as Dog Catcher. We would have a primary to see which of the 4 gets to compete in the "real" election, against their opponents from the other party/parties. In some places, where one of the major parties has little or no presence, the primary actually decides (for all practical purposes) who will finally be elected.

In some states, you have to be a registered member of a party to vote for the candidates in that party's primary. Other states allow crossovers from one party to another with some controls imposed, and some have open primary elections.