Well, I changed the Assignee property from UserList to DBList. It's not so easy to find documentation on this subject. This page helped me a little http://dev.xwiki.org/xwiki/bin/view/Drafts/DatabaseListProperties

2011/12/1 Maxime Sinclair <maxime.sinclair(a)gmail.com>om>: > Hi, > > I try to use the Todo Application > (http://extensions.xwiki.org/xwiki/bin/view/Extension/Todo+Application) > in a farm context with all the users defined in the master wiki. > > The only problem is the Assignee property defined as a "List of > Users". In a subwiki of the farm, the only defined users are > XWiki.Admin and XWikiGuest. > It would be fine if the list of users can be filled from the > XWiki.AllGroup of the subwiki. But I don't know how to do this. Could > you put me on the right direction ? May be you have a better idea... > > I hope my question is clear. > > Maxime