From the customer view in https://admin.webex.com, go to Users, click Manage Users, and select Resend invitations.
The invitation emails get sent out to users who haven't activated their accounts yet. You can close the browser or this window and your task continues to run. To review the progress of your task, click View all tasks in the Overview tab.
If there are errors that are recorded in the Tasks window, select Export errors to CSV to view the errors in a CSV file.