The novel network slicing paradigm represents an effective turning point to operate future wireless networks. Available networking and computational resources may be shared across different (instantiations of) services tailored onto specific vertical needs, envisioned as the main infrastructure tenants. While such customization enables meeting advanced key performance indicators (KPIs) introduced by upcoming 5G networks, advanced multi-tenancy approaches help to abate the cost of deploying and operating the network. However, the network slicing implementation requires a number of non-trivial practical considerations, including how resource sharing operations are actually implemented, how involved parties establish the corresponding agreement to instantiate, operate, and terminate such a sharing or the design of functional modules and interfaces supporting these operations. In this paper, we present a novel framework that unveils proper answers to the above design challenges. While existing initiatives are typically limited to single-domain and single-owner scenarios, our framework overcomes these limitations by enlarging the administrative scope of the network deployments fostering different providers to collaborate so as to facilitate a larger set of resources even spread across multiple domains. Numerical evaluations confirm the effectiveness and efficiency of the presented solution.