If you're currently working for a company, you may very well be sitting on a gold mine of potential writing experience.  You may already be growing your portfolio without even realizing it.

But always remember:  Obtain your employer's permission before including a writing sample you have done on the job in your portfolio. Some material may contain proprietary information which your company does not want publicized; it is your responsibility as a professional writer to exhibit ethical behavior. 

Once you have permission from your employer, think of what you have done at work that can be used as a writing sample:

Have you contributed any articles to your company's newsletter? That counts. As a matter of fact, a large portion of many freelance writers' incomes is generated from writing corporate newsletters.

Have you contributed to portions of a technical manual or handbook used at work? That counts, too.

Virtually anything you write for your company can be incorporated into a portfolio. Why not ask if there is something your company needs written? Volunteer. Be a star! Do a great job for your employer and build your writing portfolio at the same time. It's a win-win situation!