In the Settings session, go to General Settings and then click on the Plugins tab. Search for Members only Settings.
After activating the Members Only Website Locker, the unlogged users will be unable to access the public pages of the system, in that case an invitation will be shown on the screen for the user to become a member . OBS: The advertise page will remain public.
You can enable / disable the option for Members Only Plugin, and also adjust the level of access of the content that will be shown, which in this case can be either just for Sponsors, or both for Sponsors and visitors.
After enabling the Plugin with the desired access level and writing the message that will be displayed, you can press Save Changes.