Webex uses basic authentication by default. But if you have an identity provider, you can choose to tie that environment into Webex. Doing so lets people authenticate only once, and can then sign in with their existing corporate credentials. From the customer view in https://admin.webex.com, go to Settings, scroll to Authentication, click Modify, and then select Integrate a 3rd-party identity provider.

See this article for how to set up Single Sign-On and for all the tested identity provider solutions with Webex (such as Active Directory Federation Services, Microsoft Azure, Google Apps, and more). Once integrated, you can also suppress automated emails for new users so that you can send your own announcements.