When logged in as an Admin user:
Navigate to a host profile.
In the 'Basic Details' panel, click on the pencil icon next to the masquerade button.
Scroll down to the section titled "Choose which programs this host can see participants from:"
Here you will see a list of all the programs inside your Hanover.
Click the checkbox of the programs that you want this host to be able to have access to.
When you're finished, click 'Save'.
Once a host has access to a program, you'll need to allow individual participants to be found in the search. Here's a useful guide to help you with that.