/etc/group - the group users empty
 

I have several users who belong to the group users, everything works fine and as they should, group permissions work.. The thing is, I just noticed that /etc/group:s group users line hasnt got a single user in it.. What file determines the population of this group?

Check /etc/passwd

Each user's record in that file mentions a group id. That group is their default/initial/primary group. The user is immediately enrolled in that group when they login regardless of the contents in /etc/group.

EDIT:
Just to be thorough, the group id I'm referring to is the fourth field of each entry in /etc/passwd

ah, yes.. that explains.. many thanks.. (: