When I create some custom roles on existing sites via sitePermissionDefinitions.xml the sites displays the folowwing errors org.alfresco.scripts.ScriptException: 03120025 Failed to execute script 'classpath*:alfresco/templates/webscripts/org/alfresco/repository/site/membership/membership.get.js': 03120024 An authority was not found for GROUP_site_mynewsite_SiteCustomManager
Upon site creation a number of group Authorities are created that are used for this site. E.g. GROUP_site_mynewsite_SiteCustomManager.
If you add a new permissionGroup in sitePermissionDefinitions.xml this authority does not get created automatically for existing sites and the errors start appearing everywhere the permissions are aksed for for thiss ite.
So either you try and add this authority yourself in some way or you have to recreate your site like you suggested.
You cannot search memebers of exiting sites.
You cannot invite users to existing sites using the new custom role.