In this article we describe a system that matches learners with complementary content expertise in reaction to a learner-request for knowledge sharing. It works through the formation of ad hoc, transient communities, that exist for a limited period of time and stimulate learners socially to interact. The matchmaking system consists of a request module, a population module and a community module, all supported by a database that contains learning content, learner information and output of the system. The request module allows the learner to type in a request, the time span in which an answer should be provided and the content it is related to. The population module selects suitable learners to populate the community by determining their content competence, sharing competence, eligibility and availability. MOODLE is used to host the community. A first experiment is briefly described that shows that content competence can be successfully determined using our method. Future experiments are discussed that aim at establishing the feasibility of the overall design.