I find it much easier to edit files directly rather than remember commands like useradd -g, usermod, groupmod, etc. Is there anything wrong with just editing /etc/group directly?
Apart from not making mistakes while doing so, you have to be aware that on systems where LDAP is setup
useradd etc. can also update the data there.
Essentially if only
/etc/group, and only a limited amount of lines in there, are effected you might as well edit the file.
If you have
/etc under revision control (
git) it is easy to roll-back changes.