Do you have API for client messages

Do you have an API to access the messaging to/from clients, could this be done ?

You can use the API to quickly pull a list of all client names and emails. You could then pump that into a third party app like MailChimp for designing custom emails and dispatching to all clients.

I found this API, can i use it to send messages between companies working as a group within Quickfile ?

<System_AccountQuery  xmlns=""  xmlns:xsi=""  xsi:schemaLocation="">

No, this will just return some meta data for an account. It has no means to send messages between companies.