people do not have to register in order to post… well, at least you can set it up that way… or do you mean here?
who can do what on your forum is controlled by usergroups and permissions… please see our codex pages on that: http://codex.simple-press.com/codex/getting-started/controlling-access/
sorry, what do you mean by ‘retrieve those emails to market to’? are you talking about the emails sent our after a user registers? generally, its set out by WordPress… however, simple press does offer you a way to customize it… see forum – options – email settings if you have that enabled… if I missed your question, please restate with a bit more details…