File permissions

chmod

You can change file permissions with:

user $ chmod [r][g][u] [file]

[r] = number for root permissions, [g] = number for group permissions and [u] = number for user permissions.

7 = 4+2+1 (read/write/execute)
6 = 4+2 (read/write)
5 = 4+1 (read/execute)
4 = 4 (read)
3 = 2+1 (write/execute)
2 = 2 (write)
1 = 1 (execute)

chown

You can change owner and group of file with:

root # chown [user]:[group] [file]

You can change owner of folder and files inside recursively with:

root # chown -R [user]:[group] [folder]

gpasswd

You can add user to group with:

root # gpasswd -a [user] [group]

You can remove user from group with:

root # gpasswd -d [user] [group]

useradd

You can add new user with:

root # useradd -g users -G wheel,audio,portage -m [user]
root # passwd [user]

You can delete user with:

root # userdel [user]

groupadd

You can add new group with:

root # groupadd [group]

You can delete group with:

root # groupdel [group]