Theres 3 server i know for EU with plenty of multiboxers on

Grim Batol - Horde - <Gimp> This guild has been runing for a very long time now and has taken in alot of multiboxers in that time, there are also some members who are solo, mostly friends of multiboxers.

Ravencrest - Alliance - <Dragon Wagon> a new guild, they take on anyone who wants in so you will get a few 'wtf 5 same characters' but the people who run it like multiboxers and will defend it. so if you like a guild that has multiboxers and lots of solo people (most new to the game) then this is it.
Also came across multiboxers who are not in the guild.

Outland - Alliance - <Autobots> Im not in this guild just a little stalking when i was looking for a multiboxing server, tend to be 4 multiboxers on when i was looking, not sure if any solo members are in there.
At the moment i dont think i can make a team on here, doing RAF again and im getting serious burn out leveling teams

All 3 have a lot of multiboxers, just do what i did and stalk the servers for awhile until you find what you like.