Puppet classes are not available for inclusion in host groups until imported from the filesystem
I wasn't sure if this should be katello-integration or puppet integration.
When we add puppet modules in Katello and promote the content views, we are no longer able to view/add/include the new classes in hostgroups.
As a workaround, I need to go to Configure -> Puppet Classes and "Import" from the filesystem. After that, I am able to add the classes to the hostgroup.