FWIW, the way to do this currently is to:
- restrict “See” permission for the category to the ProjectWeek group
- make all posts into Wiki posts
- give every group member “minimum wiki edit” trust (TL1)
- then everyone in the group could edit all posts.
When the Discourse platform updates to the next release (I don’t know ETA) then it will be possible to have Wiki posts visible outside the group, but only editable by members of group with “Create” permission on the category: