This is a fairly simply achievement to obtain after you've unlocked both Baby Groot and normal Groot following the first two or three missions.
When you're returned to the Manhattan streets, hold down
and it'll bring up your character selection screen. Scroll to Groot and select him. Then switch to another player and hold down
again. Go back to Groot on the character selection screen and hit
to change him to Baby Groot.
Once you're back to street view, the achievement unlocks about three seconds later as you'll have both Baby Groot and normal Groot in your three-person party.