At Admin Panel -> Settings -> Switches, the "clickthrough to denial page" switch allows you to force would-be submitters to read about why they can't submit. When you have the "can submit topics" usergroup permission turned off for a usergroup (Admin -> Members -> Manage Usergroups), they won't normally see a link to a submission page. If you activate this clickthrough switch, however, the submitter will see the submit link like everyone else and be able to click it. Since they don't have permission to actually submit, they'll see a special page -- the "submission denial" template which you can edit at Admin Panel -> Themes -> Manage Templates -> Access Templates -> Submission Denial -- which explains to them what they need to do in order to be allowed to submit (such as register an account).