Techniacally they're not supposed to. It's where the whole "friend of mine/friend of ours" thing is meant to come into play. Though you'd have to think some guys were well aware of other connected guys, whether they were meant to or not.
Considering there's around 1000 made guys between the Five Families, it would be surprising to learn most guys did indeed know most others outside of joint ventures and gossip.
1000 made guys is a bit much, maybe max 700 and thats assuming both the genovese and gambinos have 200 and the rest have 100
