When importing users, you have the option to "Send onboarding email to imported users."
When this option is checked, all new users who receive a license will receive an onboarding email.
Important Note: Only users who are assigned a license will receive an email. If you have the LICENSE field set to FALSE in your CSV, the user will not receive a license or onboarding email.
Standard Onboarding Email
The onboarding email users receive is shown below (as long as SSO is not enabled):
SSO Onboarding Email
If your organization is using SSO, they will receive the following email: