For a user to have rights in a company, the following is required:
The user must belong to the company
Open the user in the Administration Tool under Organisation-Users and add the company to the Add/Delete companies tab under the heading User's companies
The user must belong to a role
Open the role where the rights are to be specified in the Administration Tool under Organisation-Roles and add the user to the role.
The company must belong to the role
Add the company to the role
The role must have access rights in the company
Go to The role's companies tab on the role form and select the company in question
Then go to the lower tabs and make the settings. These settings must be made at each company separately.
Read more about the different tabs in these articles: