  Given an initial resource allocation, where some agents may envy others or
where a different distribution of resources might lead to higher social
welfare, our goal is to improve the allocation without reassigning resources.
We consider a sharing concept allowing resources being shared with social
network neighbors of the resource owners. To this end, we introduce a formal
model that allows a central authority to compute an optimal sharing between
neighbors based on an initial allocation. Advocating this point of view, we
focus on the most basic scenario where a resource may be shared by two
neighbors in a social network and each agent can participate in a bounded
number of sharings. We present algorithms for optimizing utilitarian and
egalitarian social welfare of allocations and for reducing the number of
envious agents. In particular, we examine the computational complexity with
respect to several natural parameters. Furthermore, we study cases with
restricted social network structures and, among others, devise polynomial-time
algorithms in path- and tree-like (hierarchical) social networks.
