A Republican MUST file a Change of Party form and register as a Democrat to partipate in their caucus. You must be registered to vote in that precinct before you can participate. An exception is made for young people who will be eligable to vote in the General Election but are not yet 18 y.o.
There is a "Clerk" at each caucus who has a list of all the registered party voteres in the precinct, and names are checked against that list to determine eligible participants.