In this paper, we consider restricted data sharing between a set of parties that wish to provide some set of online services requiring such data sharing. We assume that each party stores its data in private relational databases, and is given a set of mutually agreed set of authorization rules that may involve joins over relations owned by one or more parties. Although the query planning problem in such an environment is similar to the one for distributed databases, the access restrictions introduce significant additional complexity that we address in this paper. We examine the problem of efficiently enforcing rules and generating query execution plans in this environment. Because of the exponential complexity of optimal query planning, our query planning algorithm is heuristics based but produces excellent, if not optimal, results in most of the practical cases.