What types of things do they do that you like?
The weekly posts are really nice for setting expectations for users. Although this forum isn't the best for a weekly post format, I think that creating dedicated topics to things like: Marketing | Art | Playtesting | Writing | Technical Accomplishments might encourage more focused spaces for people to congregate around.
People enjoy guidance, and subtopics help in facilitating that organization :)
They are all rather broad subjects. There's already an Art category with the 2D and 3D sub-boards, which is a good starting point. As for Playtesting, I think it could use its own sub-board within the Game Development category. Other things like Writing or Music could probably fall under Art as well. Marketing is a finicky topic because it's not development related. Maybe a sub-board for business related subjects?