The SearchAll module lets your users search all Wikidot sites, including private sites (if they are logged in, and for private sites that they are members of). You can place the SearchAll module itself on any page, but your site must contain a page called "search:all" that (also) contains the SearchAll module.
The SearchAll module does not allow any attributes.
On your site start page:
++ Search all Wikidot sites [[module SearchAll]] = ([http://www.wikidot.com/doc:searching Search tips])
On your site's search:all page: