Adding of friends.

[Nickfootball]

When you say server, is that the league?

[Altiplano]

No not league. Server is much bigger, thousands(?) of leagues.
You will all need to become friends through Facebook with the player(s) on different servers.

[Blast]

if you friend that manager on facebook you can then add him as friend in game

But yes stupid that you can friend anyone else but not if via association.