How can I enable the "Invite" function for all posts?

At the bottom of each post, there is a “Share” button:

I have two questions regarding this feature:

  1. Why do certain posts in my forum have an “Invite” button when clicking this “Share” button, while others do not?

  2. For posts that do have the “Invite” button, clicking it does not seem to work. What steps should I take to enable this function effectively?