I am sure that this subject has been debated already, but I couldn't find it. Who owns a post and what type of issues of this kind typically arise in forums?
Technically any content created is the copyright of the created. However it's pretty clear that by making the post the author is taking the action of allowing the rights to that content to be used by a forum. Normally a TOS on signup helps substantiate the rights and uses of content. If you own a forum it's wise to create your own TOS and Privacy Polices.
Not really, but I'm a forum member and about to start a forum, so it's wise to know how it works, I think. And sometimes there are issues between members and owners, so...
More likely then not your member dont copyright anything since they have no incentive to! When copyrighting takes place its bc there is motivation to do so.