Has there been a convention established on what the best thing to set the
groupID on the linux profile of a user to?

It seems the home directory permissions allow r_x to this group, enabling
all who are members to have access to it.

How do you keep track of the userID in the linux profile to prevent

If you leave both these fields blank and linux enable a user, does the linux
system pick them?

I can't find much in the way of documentation explaining how this stuff