Once a new user has been registered on the system, the user would typically receive a welcome email containing their login details for the Proofpoint Essentials Interface.
If they have not received a welcome email it may be because your default new user privilege is set to silent user. Default new user privileges can be found under Company Settings > Import users or SMTP Discovery.
I users were set as Silent, you can use the Mass Update feature (in Users and Group, the Select drop down ) to set these users to End-User. In addition, at the same time you can use the feature to "Send Welcome Email" at the time of update.