FriendsShare Resurrection is an AddOn that lets you keep the same friends list across characters, on a per server basis.
If you add or remove someone from your friends list and relog to an alt, that person will be added/removed from the alts friends list as well. Also, any entries on that alts list which isn't in the global list, will be added to the other characters whenever you log them in.
Basically, you never have to manually rebuild your friends list when creating alts, or worry about keeping them current on all your alts.
This all works seamlessly without any user intervention.
As an additional plus, if Blizzard decides to clean out the friends list, this Addon will automatically rebuild your friend list. This clean out happens sometimes during server maintenance. :-)
Known issue: If you try to add someone to your friends list and the server won't let you do it (say, if a Horde char tries to add an Alliance char to the list, or you misspell the name), that entry will still end up on the global list, and so you'll get the error message you got trying to add it, every time you log on. So, if this happens to you, do '/friendsshare rebuild' and the global list will be rebuilt from that char's list, and all invalid entries discarded.
This Addon was written originally by Oystein, all credit belongs to him.
Please check the main page on GitHub.