Since pl created this thread, here's part of what I posted in the older thread:
One possible scenario is that, since Michael's meeting with Johnny went so well, neither he nor anyone else paid too much attention to Johnny and his men afterward. So the men, and/or Johnny, hid out after the party. We don't know how many men Johnny had. Let's say there were four. Two did the shooting, and were immediately killed by the other two. Or there were two plus Johnny, and Johnny (or Johnny and Fredo, if you take a bullish view of Fredo's capabilities) killed the two after the shooting. Or, of the four men, two were killed before the shooting, and their bodies positioned before the shooting, so that when the bodies were found, everyone's attention would be diverted, allowing the others to escape. All of these scenarios beg the question of how anyone might have gotten away in such a short timeframe with such heavy security.