Every Republican
is invited to participate. All you have to do is show up at the caucus. If you then want to attend the County, State, and District assemblies, all you have to do is convince your fellow caucusers to vote for you as their representative at the assemblies.
And, if you can't get elected at the caucus, all you have to do is go next door and convince your neighbor to attend and support you. It's all pretty simple.