I'm a developer at Fitocracy so I figure I can answer this for you!
You can currently log your weight anytime you want, or during each workout tracked, or both.
On the website you can change your weight at any time by hitting the picture of you on the top right, clicking Account Settings in the dropdown and clicking Weight.
On the website you can also set your weight at the bottom of the track page right above the submit workout button.
On the iPhone you can edit your weight by clicking on the You tab (bottom right in the bottom dashboard) and click on the weight box (bottom left box on the page).
While we currently have logs and graphs of your workouts on the Performance tab, we do not currently offer a historical view for your weight, but this will be coming soon in the next year.