Mass Hysteria does not apply to Players

The Mass Hysteria and Mass Rejuvenation passive skills have two modifiers - one which applies to you, and one which applies to allies. The modifier that applies to allies does not also apply to you. It does apply to other players.

A single player taking Mass Hysteria gets 6% increased attack speed from the first line of the passive. They do not get anything from the second line. This is working correctly.

Two allied players in either other's presence who both take Mass Hysteria each gain a total of 12% increased attack speed - 6% from their own passive, and 6% from the other player's ally bonus. This is also working correctly.

There is currently a bug if more than two players are sharing this ally bonus with each other because a player receiving the ally bonus from two other players don't get them stacking. This will be fixed in an upcoming patch, but with just two of you playing together, you won't encounter that bug.