it seems that vixie-cron does not require user to be part of the cron group to setuo new cron jobs. I am pretty sure it was required few years ago, the Gentoo's wiki page about Cron also states that user needs to be part of 'cron' group, but one can execute 'crontab -e' and install new cron jobs without the needs to be in 'cron' group.
What version of vixie-cron?
sys-process/vixie-cron-4.1-r14, the one from ~amd64.
Any update? I found it rather disturbing that it's unconfirmed even when it's so easy to test it.
*** This bug has been marked as a duplicate of bug 607426 ***