Wouldn't it be simpler to adequately set the umask and avoid polluting the group file?
in fact, the real problem is that every user MUST belong to a group.
the "old" way of adding users (in the first linux distros) was to put all users in one group, "users". it lead to problems, so the secure way of adding users: everyone is different.
unless you run a shell access box with many users, there is no real reason to put all accounts in the same group.
This seems like it's a duplicate of
which has a better answer, describing the "User Private Groups" practice. In short: having a group for every user, combined with a few other things, makes it easier for teams to share a folder without each user having to do a lot of fidgety things like setting their umask and doing a lot of chmod'ing all the time.