If you want to be on the same server as your friends, you need to accept an invite from one of them on Facebook and start the game for the first time in FB from that invitation. If they have been playing awhile, you would be on different levels unless they take over level one when you start.

The Facebook is separate from the "Let Me Play" account. If you wanted to attach it to your FB, Nordeus Support might be able to help with that. But it would NOT be likely to be on the same server as your friends.