So at this point Empire itself doesn't deal with things like Autoscaling. That said, the Demo Cloudformation template (and the bootstrap script that kicks it off easily for you) make use of Autoscaling groups for the instances that containers are being run on.

So, in theory you could autoscale just like you always would. Monitor stats for a host, if a bunch of them start to run low on resources, kick off an autoscaling event.

That said, there's been quite a bit of talk about integrating Empire with Autoscaling, so that when, say, ECS couldn't find any instances with resources free for a task, Empire could kick off the autoscaling events for you. Could be pretty awesome :)