The Reddess tells me that Charlie Sykes has interviewed Sheriff Clarke who confirms that the Fellowship of Christian Centurions was just permitted to show up and explain who they are and when they were meeting. According to Clarke, lots of groups are permitted to do this, including the Salvation Army, Toys for Tots, Second Harvest, etc.
With this clarification, I think the union is wrong. I have spent a substantial part of the last year researching and writing on the Establishment Clause in connection with the class I teach, some of the columns I write and a big long nerdy law review article I am just finishing. Although it's a bit more complicated, I think that if roll call is a time when outside groups in which the deputies may have an interest are permitted to let the group know of their availability, it doesn't violate the Establishment Clause to allow religious groups to do it.
In fact, I think it would be unconstitutional to exclude religious groups if the forum is made available to secular organizations.