I am a little confused:

Can diferent rohan heroes join their attack despite be owned by diferent players? Can all heroes of all players join their attack? Or only the 'ranged' heroes?

Got an official clarification about how this card works, shortly after it was released. Here it is:

Forth Eorlingas only allows Rohan heroes to attack enemies in the staging area. Non-Rohan characters cannot participate in these attacks, but you may declare any number of Rohan heroes you control as attackers against a single enemy in the staging area. Your friends cannot commit their Rohan heroes to your attack because it's not their attack window. The Ranged keyword does not work during these attacks because the rules for Ranged only apply to enemies engaged with a player.
Hope you enjoy riding down your enemies with the new Rohan player cards!
Cheers,
Caleb

Ok, thanks. It is cleared now.

PD: i didnt thought the 'ranged' question.

Okay, I understand other players cannot join your particular attack into the staging area, regardless of the Ranged keyword. But then, when the time for the next player's attack comes, this next player will be able to declare their Rohan hero as attackers, separately from you, right?

My understanding is that the card effect lasts for the whole combat phase. And a combat phase covers defending and attacking by all players. So the effect of the card lasts and applies all players, but each player can attack into the staging separately from any other player. Am I right?

As long as they have rohan heroes, yes.