I find my answer, and maybe it can help some people.
It's not : #if($user.isUserInGroup('Gestionnaires'))
But, it's : #if($xwiki.getUser($user).isUserInGroup("XWiki.Gestionnaires"))
Hope it can be useful for someone else ^^
Thanks,
Stéphanie
2012/6/18 Stéphanie Roulland <stephanie.roulland(a)gmail.com>
  I've already tried that too but it doesn't
work too.
  2012/6/18 Jeremie BOUSQUET <jeremie.bousquet(a)gmail.com>
  Maybe
$user.isUserInGroup('XWiki.Gestionnaires') ?
 2012/6/18 Stéphanie <stephanie.roulland(a)gmail.com>om>:
  Hello,
 I'm trying to retrieve all users of a given group, but maybe there is a
 problem with my code.
 First, this is my code :
 {{velocity}}
 #set($users = $xwiki.rightsmanager.usersApi.allUsersNames)
 #foreach($user in $users)
  $user
  #if($user.isUserInGroup('Gestionnaires'))
    $user
  #end
 #end
 {{/velocity}}
 I have 6 users in my group "Gestionnaires" but none of these users is
 displayed with this code.
 Do you have an idea of why it doesn't work ? Do I forget something ?
 Thank you very much in advance for everyone who will answer me ^^
 Thanks,
 Stéphanie
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 Ingénieur domaine collaboratif
 02.29.05.42.23
 
--
Stéphanie ROULLAND
06.77.99.36.41
-------------------------
Contrat de professionnalisation DCNS
Ingénieur domaine collaboratif
02.29.05.42.23