When a non-member posts, one of the regulars will ping or Freepmail me or file an abuse report. And the post is removed. The caucuses have been around for so long, they usually also kindly inform the non-member of the guidelines.
In addition to the usual guidelines that apply to religious debate (click on my profile page for more) - neither the article nor the reply posts on a caucus thread may speak in behalf of non-members. In this example, if the article or a reply post spoke about Protestant beliefs the caucus would be broken and the thread opened (caucus label removed) so that the Protestants could speak for themselves.
Again, the "caucus" designation only applies to threads posted on the Religion Forum. There are no caucuses in News/Activism, Bloggers, Chat, etc.
The pro-Lifers were thinking about declaring a "pro-Life Caucus" and posting some or all of their threads to the Religion Forum. They decided against it because articles posted to News/Activism get more traffic.
Thank you very much. A very clear explanation! :)