Ubuntu – Output of getent group

The useradd command will try to add a new user. Since your user already exists this is not what you want.

Instead: To modify an existing user, like adding that user to a new group, use the usermod command.

Try this:

sudo usermod -a -G groupName userName

• The -a (append) switch is essential. Otherwise, the user will be removed from any groups, not in the list.

• The -G switch takes a (comma-separated) list of additional groups to assign the user to.

In general (for the GUI, or for already running processes, etc.), the user will need to log out and log back in to see their new group added. For the current shell session, you can use newgrp:

newgrp groupName

• newgrp adds the group to the current shell session.

Is it possible to assign a "group" to a file/folder that the file user (owner) does not belong to?

Certainly. For example:

$ sudo ls -l /var/spool/cron/crontabs
total 8
-rw------- 1 muru     crontab 1136 Jul 21 12:30 muru
-rw------- 1 root     crontab 1090 Jan 20  2015 root

$ groups
muru adm cdrom sudo dip plugdev lpadmin sambashare
$ groups root
root : root

As you can see, neither root, nor muru, are members of crontab, yet the crontab files' group owner is crontab.

Is there any restriction between file's owner and group?

None at all¹.

¹Well, there are restrictions on each individually: the UIDs and GIDs have to be 32-bit integers (IIRC), or the username and group names must exist:

# touch /tmp/foo
# chown 1000000000:2000000000 /tmp/foo
# ls -l !$
ls -l /tmp/foo
-rw-r--r-- 1 1000000000 2000000000 0 Sep 15 09:04 /tmp/foo
# chown gibberish:garbage /tmp/foo
chown: invalid user: ‘gibberish:garbage’

But, no, no relation between owner and group.