As a person who adds at least as much as he creates, I have a few tips too:
1: make it clear, right off, that adding is OK!
I cannot remember all the times that I have gone to add to a story, gotten to the page where you write the new chapter, realized that there are no contribution guidelines and said "... on second thought, maybe not..."
Similarly, the number of times I have waffled about even looking at a story, so much as opening the first page then noticed a little aside from the author in title or subtitles such as (Adders Encouraged!) and thus chosen to look, knowing that should I be moved to contribute, it would be welcome, I cannot give a proper account of.
2: Do not rely on others to develop your story.
If your story is not inspired and inspiring enough to keep your own interest, it certainly will not be for anyone else.
There are a handful of 'Angel Writers' such as myself who pick up tired and neglected stories and breathe fresh life into them, but we are a rare breed, and fickle. We may add for a day, a week, a month even, but if the story doesn't gain an audience and take off, we tire, the muse leaves us, and we abandon the work to fall asleep once more, waiting for the next person to find their inspiration in it.
The only person who can and will keep your story strong and vibrant is you. Be prepared to add to it for a long time, several chapters a day at least: keep it visible, keep it on the front page: and do so LONG after it has ceaced to be a "New Story"...
3: Expect to have to refuse chapters.
If you are an author, or even editor, on this site people WILL insist on submitting chapters that: A: don't make sense in English (and, I would infer, in Hotchduitch, Francas, Espanol, Nihongo, Guoyu, or Russkiy,) B: Are of such low quality that they should be embarrassed to have written it, C: violate the T.o.U. or D: are in contravention of your stories submission guidelines.
You MUST, at that point, refuse to allow this in your story. If you allow yourself to be swayed into publishing that kind of garbage by pity or a misguided sense of obligation you will lose all credibility and no serious authors will write for you thereafter.
Hope that helps!