I have never done this, but if you want so general advice; you can search via the normal webscript javascript search object for users and groups, if you can search for groups you can get their members and that would allow you to create the drop down. Becareful because I suspect that you assign tasks to users via a user object id and not a user name (although you can get the user object from the user id).
You have source code for this in the site members webscripts, so start by looking at the scripts called when you search for groups and users off the share site members pages.
Bob Johnson