When I get a sitemap generator to generate a sitemap it dosnt see them
Since you are already allowing users to generate content you would seem to be in the best position to generate the sitemap; not an external tool. When the sitemap changes, you can optionally ping Google.
If an external sitemap generator is unable to find these pages then it's because the new pages aren't being linked to and your site cannot be crawled successfully.
In addition to the XML sitemap, you should probably be generating some kind of HTML index/sitemap that enables the new pages to be crawled easily.
do it automatically like it does with regualr php pages
These are regular PHP pages. article.php?id=123 might sometimes be referred to as a dynamic URL, but really there's no difference.
anyone can post anything
Sounds like a moderation nightmare?! Is this content worthy of being indexed??