If enabled, projects have two areas where you can have discussions. There is an entire project related discussion at the bottom of the page and each phase has an area for discussions.
By default, new messages will not trigger an e-mail notification. You can enable this feature by following the instructions in the notifications article.
If you’d like the ability to attach files to comments consider installing the free comment attachment plugin from the WordPress repository.
If you don’t see a discussion area that means that comments have been turned off (either globally or for the project specifically.)
First, try editing the project and looking for the “Discussions” box down towards the bottom of the page. Make sure that “Allow Comments” is checked.
If you don’t see this box it means that the discussions have been hidden. Scroll up to the top of the page and click on the Screen Options tab in the top right. A bar will appear and you can unhide the discussions box.