First contact with the Roswellians happened by accident, when Buzz Lightyear's ship crash landed on the planet. Buzz and Mira Nova were found unconscious by the Roswellian military forces, which brought them to a secret base and hid the ship's remains (including a damaged XR), secretly replacing it with a meteorological instrument's remains to cover up the extraterrestrials' existence from the population. Booster meanwhile went unnoticed by the military and went to hide in a local city, befriending a small girl and her father the sheriff, who help him rescue his friends in the base and leave the planet.
Later on, Roswell was invaded and conquered by Emperor Zurg and his minions, who enslaved the population until their defeat by Buzz and Booster, who finally got the trust and respect from the Roswellian authorities. Afterwards, the species became integrated to some level in galactic society; at least able to receive interstellar news channels and travel off-planet.