List of Existing Workgroups?
In order for a prefix (xxxx:) to be recognised by the software, it needs to be added to the server file, and thus it becomes a namespace that is searchable, and pages are filed automatically into the namespace when created with a title that starts with the prefix. "Workgroup:" is actually what would be called a "pseudo-namespace" which means it resembles a namespace and provides the identification properties a member would recognise, however it does not have the functionality. See this definition by MediaWiki.
Pages with the "Workgroup:" prefix are actually in the main namespace (the one without a prefix), and "Workgroup:" is actually part of the title. This means that methods of dropping the namespace from being displayed or used in sort keys (such as a preceding colon or a vertical bar) do not work, and one has to manually indicate a sort key or display text in order to do so. It also means that in order to search only among pages that are "in" the pseudo-namespace, one should do something like include "Workgroup:" in the search parameters.