In ACP > Manage > Privileges, select the category you want to allow guests to post in, and give guests the "Create Topics" and "Reply to Topics" privileges.
Does anyone know if this is possible with the stock tools or if there is currently a plugin to acheive this?
The ability to select visibility of certain posts, for example an announcement, visible only to those in a specific group. Or a way to create categories only available to members of a specific group.
You can already create a category only visible to a certain group by creating such a group and selecting the privileges in the ACP.