Others have done it before and have succeeded with the following main arguments

Internal solutions from large companies usually have 2 main flaws: 1) It’s usually not possible to invite and create user accounts for customers and partners outside the company. 2) You usually have file size limitations if you are using regular email for your projects.

All GroupCamp applications are strongly integrated with email. And even with the best online project management tool, email remains the most used communication tool for all business users. But email as a service has its own drawbacks. and needs to be strongly integrated with your online project management to bring enhanced productivity.

Start by creating companies in the Admin section and invite users from those companies. When you create a multi-company project, add the companies which should participate. In a multi-company project you can decide if a file or a task list is private or not. If you have multiple companies you can decide which one of them can see a private file. For example if you have multiple versions of a file you can mark them as private in the initial drafts. Once you reach a version that can be shared with outside companies, if the file is no longer marked as private, it will be shared by selected companies.
To share files with your clients and prtners, simply add users from outside companies to your project to access the secure extranet (SSL 256 bits - https access) with a controlled access.