Probably the most famous aspect of the system was the introduction of the notions of process groups and multicasting. High performance communication is critical for V or any other distributed system. The communication protocols, not the software, define the system. They wanted a single protocol for all communication. They use the analogy of a software backplane, where services are plugged in like peripheral cards.
V is probably the first system where the concepts of process groups and multicasting came about.
They made the observation that entities can be categorized into groups, and messages may need to be sent to all members of a group. Look at the graph in the paper, it does a pretty good job. Basically, a local GetByte operation bounces to the local kernel, to the remote kernel, to the remote file server process, and back. They were able to make IPC fast by using synchronous IPC, fixed size messages, and optimizing for the above things. In either case, the client uses Send to send a message to server.