It seems some are conflating copyright and ownership.
One does not give up one’s copyright of authorship UNLESS one actually signs that right over; that requires a copyright transfer agreement or, in case of a company, an employment agreement that specifically states all rights for work done by employee for said company are the property of the employer. It often happens that the creator maintains copyrights, but another entity actually owns the product. This is very common in the music industry. Musicians often retain copyrights to songs, but do give up ownership of same songs for distribution and sales purposes.
In this case, a public board, authors of posts retain both copyright and ownership of their posts. CC cannot just claim those two rights because you use its board, as there is no agreement to write for CC between the posters and CC; there is only an agreement to use CC platform. However, what CC does have copyright over is the entirety of the forum, including threads and posts on its forum.
What is means is CC cannot take individual posts and use them as if the posts are property of CC, and CC cannot claim the posts belongs to it. In practical terms, this means two things: 1) CC cannot change the name of the username of the poster associated with a post to falsely indicate that CC has copyright and ownership of the post. And 2) this further means CC cannot sell the content of the posts as its original product/work without compensating the authors (posters), as the author still retains copyrights and ownership.
However, CC can use, distribute, advertise and even sell the forum as a collection of threads and posts of people discussing different topics. CC has copyright over the structure of the forum and over the collective threads and posts voluntarily placed in its forum. However, CC does not have copyright or ownership over individual posts’ content.
The only danger here really is that posters’ essays and ideas are public and subject to stealing. But, even here there is distinction to be made. An author of an essay retains copyrights and ownership, and, if plagiarized, the poster can seek legal remedies. However, one cannot copyright an idea, so if one posts a great idea and it gets copied, then there is no legal recourse.
Therefore, I also recommend you follow this advice: