@John Quile Microsoft has just released an updated autoscaling ARM Template. I haven't tried it myself, but I've been told it works much better than previous iterations. Give that a try?
You can also do it manually. Spin up a new VM using whatever image you based the host pool on, then install the RDInfra agents, as outlined here:
That usually works for us. Sometimes I can't get it to work (haven't figured out why) and say to heck with it. On those occasions I destroy the host pool and redeploy it with more hosts.