Marvel Studios has shown they're not afraid to mess with a proven formula. Take the Avengers, for instance. After the original six members of the first film, new members Vision, Scarlet Witch, Falcon, and War Machine were added in the second film. Then in Captain America: Civil War, the team broke up completely, and to this day no one can definitively say who is and who isn't an Avenger. The Guardians of the Galaxy also add some new faces to the mix in Volume 2, most notably Mantis, but does that mean Mantis is a Guardian of the Galaxy now, or are the original five the only members?
It's not like the Guardians are real strict with their membership or anything. I mean, if you're expecting Mantis to get a certificate or something that says she's an official Guardian of the Galaxy, you can forget it. But the new film does imply that she is welcome as a new member of the team, at least in an honorary role, and that's not too surprising. Comic book fans of this incarnation of the Guardians of the Galaxy, which debuted on the page in 2008, will recognize Mantis as one of the team's founding members.
Put together by Star-Lord, the original 2008 Guardians of the Galaxy team (technically the second incarnation of the team following the 1969 original) consisted of Gamora, Rocket Raccoon, Groot, Drax the Destroyer, Adam Warlock, and Phyla-Vell's version of Quasar. That was just the combat-ready portion of the team, though. Also present was Mantis, who served as the team's support staff, usually remaining at their headquarters while the rest of them went to battle. She also played a role in founding the team, telepathically coercing the other members to join at the request of Star-Lord (they weren't very happy when they found out about this).
Like many characters, her origins in the movie are considerably different than in the comics. The comic book Mantis was an Earthling who derived all of her original powers and abilities — such as her empathic telepathy — from intense martial arts training. The film keeps the telepathic powers but does away with pretty much everything else, turning Mantis into an alien and making her the assistant to Ego the Living Planet. In this form, she is extremely socially awkward with little experience interacting with other beings, but she grows close to Drax thanks to their shared weirdness.
Regardless of the character's origins, the Mantis of both the comics and the movies is clearly meant to be a member of the Guardians of the Galaxy, and I'm willing to bet she's among the team when they make their next appearance in Infinity War — especially since it's already been revealed that she's in the film in some capacity.