I remember playing an era in Mars where we wanted to communicate with our main alliance so we were told to type h/help in our normal alliance chat and were taken to another chat where even the players from the main alliance was online (20 members online)
How did they do that? And was that "legal"

I think that's just the help channel like how you have your alliance channel and it's not ilegal to talk on another chat other than world or alliance people use skype and stuff all the time

it's actually an old chat, we used to have that to ask for any kind of help which will be answered by those who knows. But it turned out useless as none check that chat anymore. So it was removed, at least it seems so.

Using that chat is not illegal, think of it like an easter egg that not many people know.

HOWEVER, please keep in mind that the chat is not member limited, as in, anyone can join the chat and see what you guys are saying. So keep that in mind whenever you're using that chat so you don't leak your infomation out to other players.