Project forums
Each project comes with a built-in forum (unless you deselect that option when editing/creating the project). The Forum is accessed through the "bubble" icon in the project bar (regardless of login type).

On the forum page, previous topics are listed, along with an option to create a new topic.

Who sees what?
Forums are meant for communication between multiple users. The simple rule is that anyone who has a job on the project also has access to the forum.
Email notifications
If you create a new topic, all matching users will be notified. To receive notifications about new posts in the topic, you can Follow (or "star") it. Replying to a topic will automatically make you follow it, but you can manually change this at any time.