One of their most significant roles from the mortal point of view is guiding the souls of the dead into the Spirit World (hence the name). However, Guides also perform a variety of other actions: they take sacrifices from the mortal world to the spirits, usher summoned spirits through the Veil that separates the worlds, and regularly appear when powerful magic is performed and at other significant events.
The Guides can be spoken to, although they are known to remain aloof from mortal concerns. Very occasionally they will give advice to mortals or spirits; most people believe that this is not down to favour or partiality, but is their way of ‘helping fate along’. Attempts to find out more are invariably unsuccessful.
As far as can be discerned they enact or embody some fundamental principle of the universe - certainly they are not material beings, or even spiritual beings in the commonly understood sense. Guides only interact with matter and spirits when they choose to do so and appear to be completely indestructable.
The Guides are the in-character avatars of the refs (the game referees) and can be identified by their loose off-white robes. They are present in the game to keep everything running smoothly, check up on what PCs are doing, and answer questions. You can talk to a Guide in-character, although be aware that they are expected to be impartial IC and OOC.
Several skills in the game (mostly magical skills) require a ref to use. If you want to use one of these skills, you can call for a Guide and one of the refs will be with you as soon as possible.