I'm tring to distribute an objects group in a plane with hair particle system, but the objects are positionated in the middle of plane and not in its surface... any advice?
The origin of the cubes in your group is in the center each cube instead of their respective bottoms. In each of the cubes in the group, move the origin to the bottom face to fix it:
Edit mode --> Select face --> Shift + S --> Snap cursor to selected
Object mode --> Ctrl + Alt + Shift + C --> Origin to 3D cursor