Create an entity for each company. Create entities in system > settings

Add a new Entity

To add a new entity:

Once the new entity is set, give users access to the scope(s).

Switch Entity

A user assigned to multiple entities can switch between entities. Click on the entity name under ENTITY in the header, the entity dialog will pop up:

Overrule Default configuration

Configuration set on Global Entity scope is applied to all entities unless overruled by the entity itself (if the checkbox is unset).