Techniques are described for managing communications between multiple intercommunicating computing nodes, such as multiple virtual machine nodes hosted on one or more physical computing machines or systems. In some situations, users may specify groups of computing nodes and optionally associated access policies for use in the managing of the communications for those groups, such as by specifying which source nodes are allowed to transmit data to particular destinations nodes. In addition, determinations of whether initiated data transmissions from source nodes to destination nodes are authorized may be dynamically negotiated for and recorded for later use in automatically authorizing future such data transmissions without negotiation. This abstract is provided to comply with rules requiring an abstract, and it is submitted with the intention that it will not be used to interpret or limit the scope or meaning of the claims.