If you do not currently have Azure Active Directory configured for users, you will need to configure it through the Microsoft Online Services Module.
Install-Module MSOnline
To enable self-service signup, you can run the following PowerShell cmdlet when connected to Office 365 via the Microsoft Online Services PowerShell:
Set-MsolCompanySettings -AllowEmailVerifiedUsers $true -AllowAdhocSubscriptions $true
After you have enabled this option, the next time users without an Azure Active Directory identity attempt to sign in to the service, they will be prompted to create a new password. After creating a password and validating their email address, they will be allowed to sign into the service. Behind the scenes, you will see new Azure Active Directory users in the portal for every user that self-registers.