A virtual network is a network to which only elected virtual machines have access. A virtual machine can have access to one or more virtual networks; for example, you could create a wider virtual network for all of your organisation's machines and then a separate one for just one or two machines to communciate.
The virtual network has been created. To add a virtual machine(s) to the virtual network you will need to create a network interface associated with the virtual network and then attach that network interface on a machine-per-machine basis. From there you will then be able to setup inter-machine communication. Please refer to the limitations section below.
At the time of writing a virtual network only creates an empty interlink between the virtual machines assigned to it and DHCP is not currently supported on associated network interfaces, although it is planned. As such you will need to manually assign an IP address and Subnet mask to virtual network interface to ensure inter-machine communication. For example, on CentOS 7 and presuming the virtual network interface is the second interface on the machine and thus occupying eth1:
Machine A: ifconfig eth1 inet 192.168.10.1 netmask 255.255.255.0 Machine B: ifconfig eth1 inet 192.168.10.2 netmask 255.255.255.0
Both machines will then be aware of each another over the virtual network.