The join bar at the bottom of a public Resource only appears when your community has a Default homepage set. If that setting is empty, the bar is hidden on every public Resource in your community.
Set a Default homepage
Go to Settings > Domains. Under Community Domain, open the Default homepage dropdown. It lists your published Custom Pages and your Sign Up Links. Pick either one and save.
The bar then appears at the bottom of the Resource reader with your community logo, name, and a Join button that sends visitors to that page.
Two things to know
The bar only shows on the Resource reader, after a visitor has unlocked it. On an email-gated or paid Resource, someone who has not yet entered their email or paid sees the access screen, which has no join button.
The row of member photos (“Join in with X members”) is separate. It needs at least 4 social proof photos configured on your community, the same ones used on your sign up pages. Without them the bar still shows, just without the photos.
Related: How to Set Up a Custom Domain for Your Community and How to Build a Homepage for Your Heartbeat Community.