I have a couple of categories that don't contain any posts (yet). What I would like to do is have a form which takes users details for future posts.
Am I right I thinking I simply edit the page which displays the current search bar..
In your category.php ( or custom taxonomy archive template ), you should add the form code in the else case of your check if there are posts
If there is no category.php you can copy and rename your index.php, and you can do the same for individual categories, e.g. category-example.php will be sued instead of category.php for the example category