Thx, this works...
 A related question to groups;
 allGroup seems to relate to XWikiAllGroup.
 How can I refer to a self created group; in my case CdlsAuthors? to add users to that
group? 
allGroup was defined right above the loop, as:
def allGroup = xwiki.getDocument('XWiki.XWikiAllGroup')
You can just use another document name.
def allGroup = xwiki.getDocument('XWiki.CdlsAuthors')
  Op 13 mrt 2010, om 13:52 heeft Sergiu Dumitriu het
volgende geschreven:
 On 03/13/2010 01:03 PM, Gerritjan Koekkoek wrote:
  Hi,
 This code from 
http://code.xwiki.org/xwiki/bin/view/Snippets/Create200DummyUsersSnippet
 for(user in users){
   fullName = user.get(0) + user.get(1)
   udoc = xwiki.getDocument("XWiki."+fullName)
   uobj = udoc.getObject("XWiki.XWikiUsers", true)
   uobj.set("first_name", user.get(0))
   uobj.set("last_name" , user.get(1))
   print("* Registering *" + udoc.fullName + "*\n")
   udoc.save() 
 if (allGroup.getObject('XWiki.XWikiGroups', 'member', udoc.fullName,
 false) == null) {
    gObj =
allGroup.newObject('XWiki.XWikiGroups')
   gObj.set('member',udoc.fullName) 
 }
  }
 How to test if membership of group Xwiki.XWikiAllgroup already exists
 If I run the script twice I get two records in the group for the same person
 I would rather have the same behaviour as with getDocument; if not exists it creates a
new one, otherwise a new version?