Scaling Scrum is easy, have a look at the Scrum guide at http://www.scrumguides.org . You have a number of teams that each operate as normal and a member from each of the team attends an additional daily scrum. Yes, it all makes sense, but it is a lot easier to say than do, even on a small scale. There are a few practical things that newer teams and organisations need to think about and do other things will descend into a state of anarchy.
By and large there are two sets of circumstances where you need to scale.