Amsterdam: group accommodation

Hi there,

It's 13 of us, we'll be staying in Amsterdam for three nights, next december
(haven't sorted out the dates yet).
I guess I'm looking for two (cheap) apartments, like two 6-people apartments
or something like that.

Can anyone help with phone numbers or websites I could check?

Thanks a lot,
Paolo.
(for a private reply, remove "dontbugme" from the email address)