You can join anything in Union. It can be a group of people by interests, a company or department in a company, some professional sphere (doctor, programmer, driver), it can be a group of your devices, validation nodes of the blockchain, or even countries.
When we create a Union there are sets of internal and external rules which defines a Union.
Internal rules define the distribution of Impact and Pressure inside a Union.
External rules define interactions with other Unions. It could be pressure rules, minimum trust/imprint access levels, events, and callbacks, etc.