Amy, we helped a client develop the following workflow:
- Marketo form on Marketo landing page filled out
- Marketo uses Webhook to request login details from the product
- Via a 'response mapping', the login details are saved in custom fields in Marketo
- Marketo sends email to new user with the login information
Note: they provided only had a temporary password, so Marketo is never storing the password entered by the user, which would be a privacy/security risk.
Note 2: the product needs to have a REST interface for account creation and return the login details via XML or JSON. Our client already had that service available, so it was easy to set up the above workflow.
The options mentioned by Val and Jace are absolutely valid too. It's probably more common than using a Marketo form. If you use their approach, it's important to also submit the Cookie ID to Marketo, so that you can track people who register via the product form.