Understood and apologies for my previous answer, next stage would depend on your business/subscription requirements and how much custom coding you would want to do.
If you prefer to avoid any coding, one of the options would be:
1. choose how do you want to map available custom service options to Liferay model, e.g. Role to Custom Service Option, or Site/Organisation to Custom Service Option
2. design look-and-feel / navigation of your portal that reflects item (1)
3. Use out-of-box sign-in/registration process
4. Make configuration that when user is enrolled Liferay automatically assigns user to default set of Custom Service Options (if any)
5. On one of the portal pages, make a display what other Custom Services available. In a case of Site/Organization mapping you can use My Communities portlet with subscription buttons available.
6. etc
If you want to have more customised business processes, the approach would be similar.
One note though, it is probably easier to implement service subscription 'after user account is created' compared to 'when user account is created'.
(We have had many cases when services are allocated when user account was created, but they are all using our own SmartForms portlet where custom actions/workflows can be executed upon form submission.)
