Update: Apparently there's a know issue about this. See this Community question: https://community.atlassian.com/t5/Confluence-questions/Preventing-Users-to-invite-others-is-it-possible/qaq-p/942643
This ability to give Trusted User permission, which allows users to invite others, is documented here: https://confluence.atlassian.com/cloud/invite-edit-and-remove-users-744721624.html?_ga=2.239764419.1769678172.1556536757-1471161920.1552492295
Where is the setting you're referring to? I'm unable to find it in our Cloud instance. I remember there being a setting about how users could be added to the system, but not an option to allow others to invite them. What controls would you have over who was getting access to your system?