Configure the Companies of your project to assign them to the tasks in which they participate.
There are three ways to add Companies to your project:
1. New Companies:From Project Settings, Companies tab, you must click on the "+ Create Company" button. A window will open where you can enter all the subcontract information, and you can also assign an image and a color label to recognize it more easily.
From Project Settings, Companies tab, you must click on the "+ Assign Company" button. A list will open with all the subcontracts registered in the company, where you can select those who will participate in your project.
There is also the option to create a subcontract directly from the Intermediate Plan. For this you just have to click on the Subcontract field in the corresponding task and then select "+ Create". The same window as in point 1 will open to add the information.
Once a company has been created for your project you can begin assigning users to that company. Furthermore, when you assign a user in the responsible column of a task, it will automatically assign their company as well.
To learn more about assigning responsibility to a task check out this article How to Assign Responsibility to a Task