Add Raspberrian (Jessie) to a wireless network

Login to the device using SSH
Type the following command:

sudo nano /etc/network/interfaces
 
Add the following lines at the bottom:

auto wlan0
iface wlan0 inet dhcp
wpa-ssid "SSIDNAME"
wpa-psk "PSKPASSPHRASE"
 
Where SSIDNAME is the SSID network name, and PSKPASSPHRASE is the passphrase for the wireless network.
EG: If my network was called:
Front Office Wireless
and my network passphrase is:
QLDWireless3827
The interfaces file will look like:

auto lo
iface lo inet loopback
iface eth0 inet manual
allow-hotplug wlan0
iface wlan0 inet manual
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
allow-hotplug wlan1
iface wlan1 inet manual
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
auto wlan0
iface wlan0 inet dhcp
wpa-ssid "Front Office Wireless"
wpa-psk "QLDWireless3827"


Press CTRL-X to exit, press Y to save changes, and press ENTER to save the file called interfaces
type:

sudo /etc/init.d/networking restart
 
to restart the networking services.

You should then be able to see the IP Address that is assigned to WLAN0.
type ifconfig to see the network settings in full.