I have tried to search for a friend, via messages, fellowships and name, without success. Is there any other way to find them? They are playing in the same world as I am. Why can't I find them? Are there multiple servers for each world, separating people?
The method Tikococo described above is the only way I know of. Click the trophy symbol next to your rank under your profile picture. Type in all or part of the persons name you are looking for in the search box and hit the search button. You should get all matching players displayed.
I find an Elven city for you in Arendyll using this method. Be sure you have your friends name correct and they are really playing the same world you are searching on and the same language server. This is the EN-International server. They use the same world names on the US-United States server.
Try getting your friend to search for you.
If that doesn't work, then, as Arjavh said they may be on the US server, or on of the other-language servers.
depends on the server, if they are on the US server, not the international one that explains it, but you could both join the same, My fs has US and Aus members