Ask HN: Companies that provide a hosted wiki?

I need to organize a bunch of information with some family members. I would like a wiki that: -is hosted by someone else -supports accounts (ie, is not publicly accessible) -makes it relatively easy to export all the content so I can back it up

I'm happy to pay for this, particularly if it means the company will still be around in a few years.

Does any one have any recommendations for a company that provides this? If a YC company has done something like this, I'd be happy to give them a shot.