Track the login count for external users to ensure that everyone with access to a tenant is logging in appropriately.
Where: This change applies to Lightning Experience and Salesforce Classic in Enterprise, Performance, Unlimited, and Developer editions.
Who: This change is available to users with the Security Center add-on subscription.
How: Monitor the External and MFA & External metrics from your Security Center dashboard via the Authentication category in Security Overview.
Assure Tenant Security by Tracking External Logins (salesforce.com)