Hi there. Well that shouldn't be too difficult as you can have your paying customers enrol via a enrolment key which you give them and you can make a course or courses just for your non-paying users where they are allowed to self enrol and just join in discussion forums. The courses with an enrolment key won't be accessible to the non-paying customers. Have you taken a look at the documentation on Enrolment? http://docs.moodle.org/22/en/Course_enrolment
Forum
Discussion forum only setup
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