[TASK] Allow existing groups to be synchronized

Resolves: #60946
Change-Id: I11e8f32eccc19d32fcbc9b400b8fc46aa417cfbb
Reviewed-on: http://review.typo3.org/32133
Reviewed-by: Thierry Brodard <thierry.brodard@techniconcept.ch>
Tested-by: Thierry Brodard <thierry.brodard@techniconcept.ch>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>