If I am not wrong, the file
/etc/group shows the list of the groups and for each one are listed the users that are members as secondary group only.
I created some users in order to do some test. The strange behavior is that my user is listed in my primary group. The other users don't appear in them primary groups. Do you have any idea? Here you are the example on my CentOS system:
[root@server2 ~]# id intore uid=1000(intore) gid=1000(intore) groups=1000(intore),10(wheel) root@server2 ~]# cat /etc/group lisa:x:1001: lisa_m:x:1002: betty:x:1009: sales:x:1007:bob,betty,linda,lisa account:x:1008:bill,beatrix,lori,bob bill:x:1010: beatrix:x:1011: linda:x:1003: wheel:x:10:intore,lisa,lisa_m intore:x:1000:intore