Setting a bundle to private will make the bundle available via direct link only. This means the bundle will not appear on the catalog page for access to content contained within the bundle, as it would if the bundle was not private.
Navigate to the bundle you want to make private under Bundles and click Edit. Scroll down and toggle Available Only By Direct Link.
In order to get the Direct Private Link you can follow these steps:
Navigate to Bundles.
Find the Bundle for which you'd like to obtain the share link, then click on the share icon to the left of the Edit option.
This share icon looks like this:
Then copy the link in the window that pops up.
Here's a gif of how you can get the share link.