Marcos,

Take a look at the Organic Groups (http://drupal.org/project/og) module.  Many other modules work with it to do what you are talking about. 

There is also the Drupal Commons ( http://drupal.org/project/commons) distribution that might have what you are looking for out of the box.

Thanks,
Steve

Hello!

This is my first e-mail, but sounds to me like a complex question.

Is possible use groups like users (send private messages, post in discussion board, etc.)?

The idea is create groups for companies and each company has multiple users to admin, but all user (in a company) need to speak in the company's name.

Best regards.

--
Marcos Barbosa <marcosestevesbarbosa@gmail.com>

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