Assigning a specific company to a user is controlled by the admin, who is managing TallyDekho Desktop App. 
On TallyDekho desktop app you have a list of companies and users accessing the data, in front of user name click on 'Assign' button to give specific company access to that user. 
Select the companies which you want to assign to the user and click 'Save & Assign'. You will get a popup of 'Successful assignment'.