If you are a tenant admin, and want to see who has signed into Power BI, you can use the Azure Active Directory access and usage reports to gain visibility. You can access the activity report within the new and the classic Azure Active Directory (Azure AD) portals.
You can use the Azure AD portal to view sign-in activity.
Browse to the Azure portal and select Azure Active Directory.
Under Activity, select Sign-ins.
Filter the application by either Microsoft Power BI or Power BI Gateway and select Apply.
For detailed explanation have a look at this.