allowing users to add subcategory

Hi all,

I’ve been playing around with WP for the purpose of setting up a fitness training website.

What I want my members to be able to do is set up individual training diaries as subcategories of a category I have named training logs.

Is this the best way of implementing this concept or have I gotten the wrong end of the stick? (Yep, I am a blogging newbie)

thanks
Ken