Giving this as a social games in the world of facebook with high degree of interactivity, I find it hard to be believe that there isn't a 'invite friend' button.
This social game have such a high degree of socializing that the devs either can't or forgotten to put the 'invite friend' buttton.

Either I there is a 'invite friend' button and i am a complete idiot.
Or I just made a really good idea

Legion --> View Friends --> Invite more friends to play

nasty place to put it

Except there no real point in asking your friends to join the game. if your legion is full or close to it they either cant join you or are so far below you they cant get your npc alerts or really interact with your game play.