Using the new Azure AD B2C service in preview, I am looking to be able to send a user an invitation email to complete sign-up for our SaaS application. Each of our customers
You can use custom policies to achieve this. You can send a JWT to B2C that includes the 'customer' the user is assigned to.
There is sample code here that sends the JWT to B2C.
The leaf policy that receives the JWT is here.
The sample app is here.
This is not a supported feature. I recommend that you use our User Voice forum to make feature requests; we are tracking those.
There is an existing feature request for this: AADB2C: Send email invitation for new user to sign up