After spending waaaaaay too long attempting to deduce the mission locations required to unlock each of the Guardians of the Galaxy, I'm going to try and make it a lot easier for others.
seven six different characters that you must unlock/purchase from the character menu and you MUST complete the main story to enable their missions to appear in the Manhattan Hub.
Here are the seven characters as well as the rough locations of each of their missions.
Drax - Mission 1: Can be found in Times Square. Appears as a picture of Drax on the map. (Completion of this mission unlocks Gamora, one of the Guardians of the Galaxy, for purchase).
Mission 2: Can be found in Times Square right around the corner from the previous mission. Appears as a picture of Gwen Stacey on the map.
Mission 3: On top of the Marvel HQ. Appears as a picture of Drax on the map. (This mission will unlock Drax for purchase).
Gamora - As mentioned before, Gamora is unlocked by completing Drax's first mission.
Rocket Raccoon - Mission 1: Slightly North of Central Park/West Harlem and South of X-Mansion in a location named Scrapyard. Appears as a picture of a garbage truck on the map.
Mission 2: West of Central Park in the Upper West Side area of the city. Appears as a picture of Star Lord on the map. (Completion of this mission unlocks Star Lord, one of the Guardians of the Galaxy, for purchase).
Mission 3: On the southeast coast of the city. Look for Stark Tower and go east and then down. Appears as a picture of Rocket Raccoon on the map. (This mission will unlock Rocket Raccoon for purchase).
Star Lord - As mentioned before, Star Lord is unlocked by completing Rocket Raccoon's second mission.
Groot - Mission 1: Located in the northern section of Central Park. Appears as the Flower Car on the map.
Mission 2: Located in the area just southwest of the Daily Bugle. Appears as Mandarin's Boat on the map.
Mission 3: Located just south of X-Mansion. Appears as a picture of Groot on the map. (This mission will unlock Groot for purchase).
Nova - Foot Race: Located in the Yorkville area of the city due northeast of Central Park. Appears as a picture of Nova on the map. (Completing the foot race will unlock Nova for purchase).
(Character is not needed for achievement to pop, thanks EllandRoad Dave and OMG Lumi for confirming)
Ronan the Accuser - Event: Directly opposite the circus tent's entrance in Central Park. Appears as a picture of Ronan the Accuser on the map, however the event is actually located underground. To get into this area go due south of Ronan's map icon towards the silver and gold statues of Stan Lee. From here hop down onto the grass next to a tree, turn around so you're facing back towards the statues and break open a secret passageway hidden by foliage. You will need a fire based character and an electricity based character to complete the event within. (Completion of the event will unlock Ronan the Accuser for purchase).
Once you have unlocked and purchased all of the above characters, the achievement will pop.