Ghost Rider has existed for many years and in many different forms in the Marvel Universe, with the Spirit of Vengeance empowering countless heroes and villains over Marvel's long history. Of course, while Earth has had dozens of Ghost Riders to protect it from supernatural threats, there's no reason this particular hero should be confined to one planet - and the original Guardians of the Galaxy showed they aren't!
Technically speaking, two versions of Ghost Rider have worked with the Guardians, one of which was the Punisher himself, Frank Castle, in the form of the Cosmic Ghost Rider as shown in Guardians of the Galaxy Vol. 5 #1. While the Cosmic Ghost Rider is the most recent Spirit of Vengeance to ride with the Guardians, another Rider fought alongside the cosmic heroes long before Castle was imbued with either Hellfire or the Power Cosmic. In Guardians of the Galaxy Vol. 1 #13, set in the 31st century, on Earth-691, the Guardians of the Galaxy already had their own Ghost Rider.
Wileaydus Autolycus was a priest before taking on the role of Ghost Rider; the last of an outlawed faith on his planet of Sarka. A new religion known as the Universal Church of Truth took over his world and, in so doing, effectively wiped out any and all traces of other religious powers. How Autolycus gained the power of Ghost Rider is unknown, though given the nature of how past Ghost Riders reached their demonic potential, it can be surmised some infernal force approached him after the implied genicide of Autolycus’ people and beliefs. No matter the specifics of how he first became Ghost Rider, Autolycus used his demonic powers to fly across the cosmos with the goal of eliminating every remnant of the Universal Church of Truth throughout the galaxy.
In Guardians of the Galaxy Vol. 1 #13, the Guardians are leaving a space station after making repairs to their ship; a space station belonging to the Universal Church of Truth. Ghost Rider believes the Guardians are with the church and rides his Hellcycle out into space to meet them, though his immediate intentions are none too pleasant. After a brief squabble involving laser cannons and hellfire blasts, Ghost Rider becomes an ally to the Guardians of the Galaxy once he realizes they aren’t with the religious extremists but instead are heroes who free oppressed worlds such as Autolycus’.
After their initial misunderstanding, the Ghost Rider of the 31st century often worked alongside the Guardians for the greater good of the galaxy, including one occasion where Ghost Rider and the Guardians fought against the dread Dormammu himself in Guardians of the Galaxy Vol. 1 #36. While not an official member of the team, Autolycus made himself available time and again to the Guardians of the Galaxy when the situation called for his inclusion. It's an inclusion that makes sense in the context of Marvel's wider universe, in which the roles and powers of many heroes - from Doctor Strange and Iron Fist to the Black Panther and Ghost Rider himself - have been passed down through history and, by implication, on into the future. The Ghost Rider of the future used his demonic abilities to fight for more than just his own personal cause of enacting vengeance on an oppressive religious regime, but for the greater good of the entire galaxy, making him worthy of the title of Ghost Rider and a powerful ally to the Guardians of the Galaxy.