Initially roles are database specific and apply to all the companies in that database. But you can limit roles to specific companies.
The following procedure limits a role to a specific company for a database login, but the same procedure can also be used for Windows logins.
Click Tools, Security, Database Logins and the Database Logins window appears.
Select the database login in question and click Roles and the Database Logins window for this role appears listing all the roles that have been assigned to this login.
Select the role that you want to make company specific.
In the Company field, click the AssistButton and the Companies window appears.
Select the company to which this role should apply and click OK.
Close the window and agree to rename the record.
You must create a separate line for each specific company for which you want this role to apply. If you specify that the permissions a user has only apply to a particular company in the database, the user in question will only be able to see that company.
Giving Roles to Database Logins
Modifying Permissions for Roles
Assigning Permissions to Multiple Objects