1. Any new node can connect to any running node in the blockchain, whether it's the first node, or a subsequent node. Each node has a "node address" which can be used by other nodes to connect. The node address is shown on the command line and accessible through the getinfo API.
2. Any transaction (whether relating to permissions, assets or streams) is immediately propagated to all nodes automatically.
3. There are a bunch of commands/APIs for managing key pairs and their associated addresses – please see the API documentation page. To start with, each node automatically generates a single key pair.