group files that you've posted, there is no group called
core on your system. There is a user called
core, with user ID 500, and whose primary group has the group ID 500. The group 500 has no name given in these files.
It's possible that group 500 has a name given in another database. The databases for users and groups are configured in
/etc/nsswitch.conf. You can query them with the
getent group 500
usrfiles, which appears to be a CoreOS extension. It works like
files, but looking under
/usr/share/baselayout instead of
files usrfiles means that
/etc/group will be consulted, then
Users and groups are usually not created on the fly, at boot time or otherwise; they are persistent system configuration. You could call commands like
addgroup, etc. from a Systemd unit, but that is fairly rare. I'm not familiar with CoreOS, but I highly doubt that creating a user or group called
core at boot time would achieve anything useful. If you want to give group 500 a name, create an entry in
addgroup. If you want to create a user or a group for your own purposes, use
addgroup, and don't reuse a name or number that's already taken.