[FEATURE] #5132 Add FrontEndUser.addUserGroup

Change-Id: I3470ee1d4b183d67836a0533d501969cedc7bd9e
Reviewed-on: http://review.typo3.org/32808
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>