The Republican primary is on February 29, to the voter registration rolls closed February 1. It says that the voter registration rolls are closed during that time. As a legal matter, therefore, anyone who applies during the month of February should be ineligible to participate in the Mass Meeting. Because they want the same rules for caucuses as they have for primaries. In fact, though, it is more inclusive to have a caucus, because there is no 28-day cutoff for registration for a caucus.
Some Democratic parties get a printout of the voter registration list, and check people off.
I remember in 1996 for the presidential mass meeting, we had the City registrar standing by at her office, ready to check the official list if there were any challenges to participants. Ask the Loudoun County Democrats. They have been plagued by Lyndon LaRouche supporters for years. In fact, that was our concern in Charlottesville in 1996. In Loudoun, they get a voter registration list and check everyone off.