- As of yet Unknown
As of yet, no known Signamancer has been introduced, but apparently Transylvito has some.
While no actual Signamancers or Signamancy has appeared in Erfworld canon yet, we can infer some things about the discipline's nature from indirect comments on it. Rob Balder has said about Signamancy, "Signamancy. Like the Peace sign, man. Like the protest sign, man. Signs, signs, everywhere a sign. Far out." The precise meaning of this is unclear; however, he is drawing a direct link between a visible symbol or word and a desired result. This is further strengthened by the nature of Signamantic philosophy - that all physical appearances are simply symbolic manifestations of the underlying "truth" of Erfworld.Erf-b1.5-p037 Based on these, it seems that Signamancy is in some way concerned with the relationship between "real" things and the symbols that represent them - the relationship between the map and the territory, so to speak. Signamancy is also a Hippiemancy, which may indicate certain things about the flavor and details of the discipline, but since no other Hippiemancy is adequately described as of now, it is hard to draw concrete conclusions from this connection.
It has been speculated that Parson Gotti (AKA Lord Hamster) is a Signamancer (based on Signamancy being Fate-aligned and Janis's claim that he is a Hippiemancer of some sort), however, there is far more evidence that this is true. It is possible that Janis's claim is based on Parson's true nature, that is he is a gamer as opposed to a playing piece in a game. Paradoxically as both an outsider and an expert, the significance of many aspects of Erfworld are quite clear to Parson in that he naturally looks to what everything in Erfworld signifies not what they "are". Parson conducts many signamancy experiments. These are countless, but examples are fun: When intoxicated, he decides to take a short flight across Gobwin Knob's quad. As a powerful signamancer, he is deciding to risk his life in a signamancy experiment to see what happens when a flying unit that is mountable, a dwagon, is ridden by a non-flying garrison unit with zero move. He has conducted and continues to conduct many such signamancy experiments, such as what happens when you leave a hex with zero move, attempt ranged attacks with no unit special ranged attack, blowing up Gobwin Knob, etc. With each experiment, he has proven his understanding of Signamancy, aka game rules, are real and can be taken advantage of, for example he setup chains of ranged fliers to cross hundred of hexes 'pony express style' in a single turn, defeated an entire army by decroaking a mountain, and even disobeying a direct order from Stanley without disbanding. And in case you missed the italics (above), it is also important to note that like archons and all such special units, Parson's power appears to be natural, that is he appears to be a natural signamancer. As such, Janis's claim that he is a "Hippiemancer of some sort" seems sound and more to the point, he appears to be an extremely powerful natural signamancer. This is perhaps often missed by fans of Erfworld perhaps because in this respect, they share Parson's powerful natural signamancy with regard to how they interpret Erfworld. Because after all, signamancy is all about interpretation of signals to seeing the reality that underlies what is taken for granted by others, much like Enlightenment  in our reality. (This also might explain why its called Hippiemancy.)
It has been noted by fans that Parson has created in Gobwin Knob a set of street signs based on characters from the Simpsons and his favorite foods and if Parson is a Signamacer, then the thought goes that these signs may begin to affect the streets on which they are located. In terms of mechanics, it will interesting to see if signamancy magic works to directly change the content of Erworld's reality like other magics (as the assumption above implies), or like Enlightenment , it simply impacts Parson's view of reality which then allows him to take extremely powerful "non-magical" actions that no one else would suspect possible because of what they've taken for granted.
Real World References
The Signamancy is highly reminiscent of Plato's philosophy, espoused in the Allegory of the Cave, in which Plato explained how perceived reality is nothing more than the shadows of the true reality. Also, of the Enlightenment/Awakening in general .