We examine the problem of multiple sources transmitting information to one or more receivers that require the information from all the sources, over a network where the network nodes perform randomized network coding. We consider the noncoherent case, where neither the sources nor the receivers have any knowledge of the intermediate nodes operations. We formulate a model for this problem, inspired from block- fading noncoherent MIMO communications. We prove, using information theoretic tools, that coding over subspaces is sufficient to achieve the capacity, and give bounds for the capacity. We then examine the associated combinatorial problem of code design. We extend the work by Koetter and Kschischang  to code constructions for the multisource case. Our constructions can also be viewed as coding for the noncoherent multiple-access finite-field channel.