Managing external guest users

Guest users are people from outside your organization. Anyone with either an Office 365 account in a different tenant or a Microsoft account can be a guest user. Good examples would include customers, vendors, independent contractors, and users from connected tenants. Technically speaking, while guest users do have accounts in your Azure AD, they're treated quite differently than normal users.

As you can see, there's no interface in Office 365 for creating guest users. Your choices are to head on over to the Azure AD portal, or create a guest invite through sharing in either OneDrive for Business or SharePoint:

Guest users

However, Microsoft did make it easy to find and delete guest users. That's good, because cleaning up guest access in Office 365 has been a big security challenge for a long time.