Actually, only six of the seven men were companions of George "Bugsy" Moran. The seventh person was a friend of one of the men. A very unlucky friend.
Also, two of the gunmen were dressed as police. That is the reason the victims complied to dropping their weapons and putting their hands against the wall.
Bugsy was across the street when the gunmen entered the garage, and didnt go in because he thought that the men were officers. Lucky little break.
Another interesting fact, on St. Valentines Day seven years later, Bugsy Moran had the main person in charge of the massacre shot in a bowling alley.