- As of yet Unknown
As of yet, no known Signamancer has been introduced, but apparently Transylvito has some.
Hippiemancy is the magic of relationships: detecting and altering the forces of opposition and attraction which bind people or units. 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.
As Signamancy supposedly deals with the relation of what is seen and what is there it's likely that Signamancers are at least good at psychology if it's not actually included in the abilities.
Signmancy may have to do with refusing the intangible rules, as in a protest with signs where units seeing it are caused to have to 'roll against loyalty/duty' and on a fail be able to disobey orders without popping out of existence.
Real World References
Also, analogous to Enlightenment/Awakening in general  particularly in light of Signamancy being an discipline of Hippiemancy with a sister discipline of Flower Power. For example that "The Signamancers claimed that all Stuff was Foolamancy." Erf-b1.5-p037 (note capitalized "Stuff" and "Foolamancy") is perhaps a paraphrasing directly the phrase "the world is illusion" as it is often expressed from an Enlightened perspective.