Kristi, in system setting 7.6 ( https://app.clarizen.com/Clarizen/Settings/DefaultNewUserLicenseType.aspx) you can change the default type of new user to Email. This still allows users to select a different type on a case by case basis, but will be the default behavior. If needed, you can use customizations to completely "lock down" which users (or which user types) can invite which kind of new users.
Thanks
Ariel
Comment actions