how to change user group ?
 

what command from the CLI should I run to change the group a user is in ?

usermod:

-(~:#)-> usermod
usage: usermod [-u uid [-o]] [-g group] [-G group,...]
[-d home [-m]] [-s shell] [-c comment] [-l new_name]
[-f inactive] [-e expire ] [-p passwd] [-L|-U] name

Given the parameters $user and $grp, this line will give you a comma seperated list of groups that the user will belong to after $grp is removed. I believe that is what the -G option needs.
The line uses the groups command to print a groups of lists that the user belongs to. It is of the form:
user : group1 group2 ...

the sed command removes the initial 'user :' , removes the group from the list (as well as a leading space, and lastly replaces the spaces with commas.

If you just wanted to add a user to a group;
bash:~# adduser whoever src
adding user whoever to group src...

thank you all for the quick answers. I also found out that I can edit the /etc/group file. is that correct?

I have one more question. About the default user groups that are created on linux. How can I find what rights has each group ?

Groups do not refer to any permission right.
Permissions are "features" of files.
ie
ls -l output can be something like
-rw-r--r-- 1 root group1 5166 Apr 24 22:53 lspci.txt
where
leading - means it is a normal file
rw- means root has read write permissions
r-- means group1 has read permission
r-- means others have read permission

In this case the user "user1" included into "group1" group will have only read permission
Hope this helps
Ciao

but what if two groups have different permissions on a file, who can I see what each group can do to a file ? if i want group1 and group 2 to have read permissions group3 to have read and write permissions and group4 and group5 to have read write and execute permissions. Can I make these settings ? if I can, how could i view these settings ?

Maybe this will help clear things up.