The Guild is a group of so-called heretics that don't worship the goddess of the land they were born on. The Guild was created due to the purge in Leanbox against heretics 10 years ago. Some of its many members include IF, Financier, and Ganache. There heretics have their own city in Lowee called Guild City. Many of the members keep the goddess they worship a secret. People look down on Guild members due to them being branded as heretics so Guild members typically keep it a secret.
In Hyperdimension Neptunia mk2 and Hyperdimension Neptunia Victory, the Guild acts more like an agency that fulfills quests for clients unlike its counterpart in Hyperdimension Neptunia. The Guild is where you can receive miscellaneous quests. Quests from the Guild affect the shares from each nation and surrounding towns and cities. Some quests are required to be finished in order to advance in the story.