Krdytkyu

I run the command

usermod -a -G dialout amashreghi

but I get amashreghi doesn't exists, however, when I try to add the user using adduser

adduser amashreghi

It says that amashreghi already exists. What's going on?!

Give this a try. (I'm assuming that you're ready to delete everything associated with this user and start over, don't run these if you're not.)

userdel -r amashreghi

grep amashreghi /etc/passwd

If that second command returns anything, run:

sudo -H gedit /etc/passwd

Remove the line that begins with amashreghi, and save the file. Try adding the user again now.

To edit /etc/group directly use vigr

From man vigr

NAME

       vipw, vigr - edit the password, group, shadow-password or shadow-group file



SYNOPSIS

       vipw [options]



       vigr [options]



DESCRIPTION

       The vipw and vigr commands edits the files /etc/passwd and /etc/group, respectively. With the -s flag,

       they will edit the shadow versions of those files, /etc/shadow and /etc/gshadow, respectively. The

       programs will set the appropriate locks to prevent file corruption. When looking for an editor, the

       programs will first try the environment variable $VISUAL, then the environment variable $EDITOR, and

       finally the default editor, vi(1).

Hence, you can edit the /etc/group file with

sudo vigr

The format of group entries can be found in man.
From man group:

NAME

       group - user group file



DESCRIPTION

       The  /etc/group file is a text file that defines the groups on the system.  There is one entry per line,

       with the following format:



           group_name:password:GID:user_list



       The fields are as follows:



       group_name  the name of the group.



       password    the (encrypted) group password.  If this field is empty, no password is needed.



       GID         the numeric group ID.



       user_list   a list of the usernames that are members of this group, separated by commas.



FILES

       /etc/group

Just do this:

sudo vim /etc/group

and add your name to the desired group, like:

somegroup:x:25:yourusername