adding user to a group
 

I created some users and later on I created a group where I'd like adding those users. In this Practical guide to linux I bought is not very clear how to do that. Is just a matter of changing the GID in the passwd file?
Please, help and patience is appreciated

check out the man pages, but it looks to me like it is

useradd -G <group name> <username>
could be -g (not really clear, just play around with it)

in RedHat the group is stored in /etc/group

Welcome to LQ.

That is only the users primary group. To add a user to a normal group do it in the group file:
/etc/group

To have joe and jack inthe juser group with gid 510:
juser:x:510:joe,jack