I really hope the name of the guild is not Companions that would be awful. As much as I hate the idea of just calling something "The so-and-so Guild" I'd much rather them call The Fighters Guild than Companions, that sounds really dumb.
http://en.wikipedia.org/wiki/Companion_cavalry
Also, the word Companions could have been chosen because NPC's that follow you are companions, and this being a mercenary for hire organization, the word is fitting.