Hi David,
You can provide an all-access subscription to users who create an account & sign in by using a direct entitlement service with DPS 2015. In this example, users who create an account with you would then sign in to the application and the application will make calls to your service to verify the user's authentication and inform DPS which content products the user is entitled to see -- in this case, all content products because you want to offer an all-access subscription.
For more information about working with Direct Entitlement services: Digital Publishing Solution Help | Set up direct entitlement in DPS 2015
Kristy