If users are part of the project they will see this discussion by default. However, you can change this if you refer to my screenshot (see bottom right "This Project") where they can associate the discussion for a particular group or user. It also depends on the licenses of the users as well. If they are full users, by default, they will see the discussions. With Clarizen’s ‘Enhanced Permissions’ setting, (Settings > 8.22) organizations can even control the viewing access of Full users, restricting them only to projects where they have a role, which is yet another required layer for large organizations with stricter confidentiality requirements.