Install Smaba:

sudo apt-get install samba

Open Samba configuration file:

sudo nano /etc/samba/smb.conf

Add at the end the lines:

[public]
    comment = This is shared folder
    path = /path/to/your/folder
    public = yes
    only guest = yes
    writable = yes
    printable = no
    read only = no

This means that you will share your  /path/to/your/folder under the name of "public" and users can write in this folder.

More info:

• Setting Up Samba

• How to setup folder share through the terminal