⟩ What is the role of SPWeb.EnsureUser method?
SPWeb.EnsureUser method validates whether the specified login name belongs to a valid user of the website or not. If the login name does not exist then it adds the login name to the website.
Eg:
Dim instance As SPWeb
Dim loginName As String
Dim returnValue As SPUser
returnValue = instance.EnsureUser(loginName)