An unnamed, unseen Magic Guild is mentioned to exist by Sneff. It may be a 'Guild' in the simplest of terms, as Sneff mentions finding simple work in context. It may be the only reference to real magic in Chrono Cross apart from Belthasar's note that he used magic in the creation of Chronopolis.
Sneff: This can't be...! Are you the rising star off the Magic Guild, Guile...!? What are you doing in a place like this!? Iff you're looking ffor work, I've got some great connections! Guile: Hmph... I have no interest in simple hand tricks... I seek only enigma beyond human perception... Sneff: Nuff! Every magic has a trick! That's what makes it interesting!
Some speculate that since Guile was going to be Magus at one point during the game, the guild might have been a troop of magic users he had formed. Considering Sprigg is a true member of the Mystics, this is an attractive idea. She also knows of the Bend of Time, where Ozzie, Slash, and Flea reside. These are interesting coincidences.
Theory and Analysis
To avoid needless disaggregation, theory on the Magic Guild's nature is presented here. So, what's the nature of the Magic Guild?
The topic of the "Magic Guild", as it's called by Sneff, is difficult to talk about because Guile is at the core of it. Guile, Sprigg, Sneff and the Fortune Teller from Termina are all part of that Magic Guild, and Sprigg (who is a Mystic) apparently knows of the Bend of Time... the place where Ozzie, Slash and Flea lurk, and where Spekkio unlocks magical ability. In my opinion, all these details make it highly possible that this guild could have just been Magus's "party" when Guile was still meant to be Magus. Guile's identity was obviously changed, but how much of the rest was changed?
Sneff states that Guile is the "rising star" in the guild, which means Guile probably wasn't its creator, though. Perhaps some Mystics simply decided to teach some humans magic at one point and this became the Magic Guild. This might have happened after Bandeau decided to create the Truce-Medina Ferry in 1000 A.D., as seen in CT.