Mejis
In Wizard and Glass they are stationed in Mejis by John Farson to assure the shipment of several tankers of oil for Farson's fighting machines, and also with looking after a Wizard's Glass, left with Rhea of the Cöos for safe keeping. Their operations are covert in nature, with the most powerful men in Mejis aware of their true purpose. The ranchers, landholders, dignitaries and law enforcers of Mejis work in collusion with Jonas' men (and ultimately John Farson) in exchange for peace with Farson after his inevitable (in the opinion of the leaders of Mejis) victory over the Affiliation and the Inner Baronies.
Steven Deschain has, though, sent his son Roland (under the alias "Will Dearborn") to hide out in Mejis for safety reasons (being such a young and prominent gunslinger, it is feared that Marten Broadcloak has taken a particular interest in destroying Roland). Roland and his ka-tet of Alain Johns and Cuthbert Allgood (also using aliases) use a cover-story that they are in Mejis on an accounting mission for the Inner Baronies. This cover is quickly seen through by Jonas, and is confirmed as such during an altercation in the local tavern of Mejis, the Traveller's Rest. The nature of the altercation involved Sheemie Ruiz, the tavern's servant-boy, spilling the contents of a slop bucket on Roy Depape's boots, and Roy's immediate threatening of Sheemie's life at gunpoint. Coincidence has it that Cuthbert and Alain were passing by the tavern just at that moment, and soon a Mexican standoff of sorts quickly forms. Jonas, and finally Roland, arrive on the scene, and during the showdown Jonas witnesses the prowess of the three young men of Roland's party and surmises they are indeed princes of the Inner Baronies and gunslingers-in-training (that Roland is already a gunslinger is a fact Jonas won't allow himself to concede).
Read more about this topic: Eldred Jonas