One can set up their private network using many different methods. One method is to place a WEP on one's network, to at least prevent casual users from entering.
extranet
A public network is a network to which anyone can connect. The best, and perhaps only pure, example of such a network is the Internet. A private network is any network to which access is restricted. A corporate network or a network in a school are examples of private networks.
VPN (virtual private network)
the answer is virtual private network
the answer your looking for is Virtual Private Network
Tunneling, also known as "port forwarding," is the transmission of data intended for use only within a private, usually corporate network through a public network in such a way that the routing node in the public network are unaware that the transmission is part of a private network. Tunneling is generally done by encapsulating the private network data and protocol information within the public network transmission units so that the private network protocol information appears to the public network as data. Tunneling allows the use of the Internet, which is a public network, to convey data on behalf of a private network.
Yes you can legally join an unlocked network. If they wanted to keep there network private and didn't want any one to use it they could stop the network from broadcasting or put a password and lock the network.
VPN (Virtual Private Network)
Private Profile
A network bridge joins two networks to make one. A network repeater extends one network to make it larger/more powerful.
YES
A Virtual Private Network (VPN)