This has not been implemented in mojoPortal.
You should be able to build a feature to at least handle adding a user to the proper roles, after they've paid, by creating an Order Completed Event Handler. There's no documentation for creating such a handler at this point but as this is a new feature of the WebStore but I've pasted in the comments from the source code. We'll have documentation on this soon. The OrderCompletedHandler was added to mojoPortal in release 18.104.22.168 (Dec 1, 2011).
If you sell products that require some kind of activation code, you could implement a custom handler to check the order for the product and if needed generate an activation key and email it to the customer.
You would implement your own handler in your own class library project, compile it into a dll, put it in the /bin folder then create an xml file to declare your handler and put it in the /WebStore/OrderCompletedHandlers folder. You will find an example xml file for this donothinghandler in that folder so you can see how to declare yours.
NOTE: All existing providers will be called for all orders after the payment is completed, not upon order creation for example if a user checks out with PayPal or google we will first get notification of the order having been received at that point an order is created but its status is OrderReceived. after the payment actually clears we get another notification from PayPal or Google and at this point the order status will be set to fulfillable and the ordercompleted event handlers will be fired, passing in the order as an event argument. It is also possible that the order could be cancelled at PayPal or Google before payment is cleared, therefore we wait until payment has cleared before firing the order completed handlers.
If you're interested in someone creating the event handler for you, give me a shout: https://i7media.net/mojoportal-request-quote.