The best way to demonstrate your editorial abilities is to show examples of documents before and after you edited them, but often that’s not an option. The best way to expand your skills is to work in the areas you’re interested in, whether that’s expanding into comprehensive editing from copy-editing or working in different subjects, but you may get caught in the common bind of not getting work because you don’t have the relevant experience. How can you get the experience and be able to demonstrate your abilities to others?
If you have an interest in computer software, a great way to gain experience and develop a portfolio of work is to volunteer as an editor with an open source documentation project. Many projects, both small and large, need the services of an editor. Some projects have experienced writers on their teams, but many rely on the programmers and volunteer enthusiasts to produce help, user guides, FAQs, tutorials, and other documentation. As you might guess, the quality of writing, and the level of English competency, varies widely. Many projects are written by people whose first language is not English.
Another variation is third-party documentation (books, tutorials, etc) written by people who are not on the software team. In both cases the results are generally made available for free, either on wikis or as downloadable PDFs; some may also be provided as printed books for a low cost. Most, if not all, are licensed under one of the Creative Commons licenses or the GPL (General Public License) or similar. Under these licenses you can keep a copy of the draft before editing and the result of your edits and show copies to anyone, thus giving you the perfect set of before and after examples to use in a portfolio, as long as you comply with any requirements of the license.
Looking for examples? I am the (volunteer) co-lead of the documentation project at OpenOffice.org. We produce a range of materials; I mainly work on the user guides. We are always looking for more writers and editors. Your time commitment is entirely up to you.
For third-party documents, you could try FLOSS Manuals (FLOSS = Free Libre Open Source Software).
Legitimate projects are welcome to add information in the comments.