Cloud radio access network (C-RAN) as a promising and cost-efficient cellular architecture has been proposed to meet the increasing demand of wireless data traffic. The main concept of C-RAN is to decouple the baseband unit (BBU) and the remote radio head (RRH), and place the BBUs in a data center for centralized control and processing. In this paper, we study the resource sharing problem in a fronthaul constrained C-RAN, where multiple service providers lease radio resources from a network operator to serve their subscribers. To provide isolation among different service providers, we introduce a threshold-based policy to control the interference among RRHs, and define a new metric to provide minimum resource guarantee for service providers. By leveraging a mobility prediction method, the user locations are predicted for traffic demand estimation and interference control. We propose a multi-timescale resource sharing mechanism, which consists of a global resource allocation process and multiple local resource allocation processes that are performed at different time scales. Simulation results show that the proposed mechanism achieves efficient resource sharing and isolation among service providers.