For Complete Case Study: http://joomla-membership-software.blogspot.in/2012/06/payplans-case-study-provide-access-to.html
Example like any dating site in which registration process is free but to get some extra features, user needs to pay. Having 2 groups: free and paid. Free users register to site by the normal process with jomsocial. Having some features limitations to free group while to have extra features user must upgrade his/her plan to paid group.
Now if user tries to access these restricted feature, he/she will be prompted to subscribe to paid plans. They will have an option to upgrade to be on the Paid group for 1 year, 6 months or 3 months.