I would have expected you could create a separate lifetime subscription product for a group with no expiry date.

Anyway, if the above is not the case, on to your job post.

For c5.6 I have a package (not in the marketplace) that adds a subscription layer on-top of the core team eCommerce addon that can do what you need. With the 2 addons combined, it becomes an expensive option if all you need is a few simple subscriptions, but that may still be a less expensive route for you than custom development.

A brief synopsis of my package:
- Configurable subscription durations as product options
- Subscription expiry and reminder emails managed by scheduled jobs
- Multiple reminder messages at intervals
- Layered on top of eCommerce, so integrates with a shop of physical products in the same cart
- Takes advantage of eCommerce for further addons such as discounts, payments, similar products (and Magic Data) etc.
- Dedicated subscription dashboard pages to facilitate subscription setup and management
- Log of subscription actions and events

Please PM me if you would like to discuss further