If you don't take steps to secure your powerline network, then yes, someone else could attach their adapter and share your network. However, you can create a secure network by pairing your adapters together, using encryption to secure the traffic, prevent eavesdropping and prevent anyone else from joining your powerline network - very similar to securing a wifi network. Assuming that your adapters support this, of course.
The power wiring can be used by multiple networks without devices on each network having access to each other, but as this is a shared access medium, this may reduce the throughput on each network. The more adapters that share the wiring, the more the "bandwidth" has to be shared. It's a bit like wifi in that respect also.
SD