There's no real reason for this in terms of making the move more plausible, but including the Guardians of the Galaxy in a Civil War movie is a ready-made way of both bulking up the numbers to make the event worthy of the word "war" and, simply put, making it even more popular by drawing in the crowds who loved their recent movie debut. The Guardians are set to come to Earth to assist the Avengers in the third Avengers movie, when Thanos will be launching an all-out assault with the completed Infinity Gauntlet. As a result, they may end up staying there - or at least setting up an Earth-base for any instances in which they happen to be there - and, consequently, they would also be expected to sign up for the Superhuman Registration Act. As outlaws, it's very easy to imagine them siding with Captain America on the anti-registration side, while another group of heroes like the Inhumans could balance things out on the other side. Imagine, Captain America, some fellow Avengers and the Guardians of the Galaxy versus Iron Man, some other Avengers and the Inhumans! Mouth-watering.
