In this paper, we discuss User Centric Networks (UCNs) as a way of, if not completely solving, considerably mitigating the problem of sharing limited network capacity and resources efficiently and fairly. UCNs are self-organizing networks where the end-user plays an active role in delivering networking functions such as providing Internet access to other users. We propose to leverage the recently proposed Software Defined Networking (SDN) paradigm to enable cooperation between wireless nodes and provide capacity sharing services in UCNs. Our SDN-based approach allows to extend coverage of existing network infrastructure (such as WiFi or 3GPP) to other end-users or ad hoc networks that would otherwise not be able to have access to network connectivity and services. Moreover, the proposed SDN-based architecture also takes into account current network load and conditions, and quality-of service (QoS) requirements. Another important feature of our framework is that security is an integral part of the architecture and protocols. We discuss the requirements for enabling capacity sharing services in the context of UCNs (e.g., resource discovery, node admission control, cooperation incentives, QoS, security, etc) and how SDN can aid in enabling such services. The paper also describes the proposed SDN-enabled capacity sharing framework for UCNs.