The DeSantis campaign (his wife, I think) called for neighboring states supporters to get out and caucus. At least, that’s how it was reported - and I do not trust reporters, in general.
Trump is doing the same. Supporters from anywhere can go to a caucus to persuade people. They just can’t vote if they’re not registered.
This is why I prefer primaries. More people can participate, and emotions aren’t the basis. Democrats would love Election Day to be run as a caucus.
She asked people from other states to participate in the Iowa caucus.