How do I enable the "Other" user for login with Active Directory?

Edit /etc/lightdm/lightdm.conf and add the following line :

greeter-show-manual-login=true

Then restart lightdm :

sudo service lightdm restart

You can also hide all users so that you enter whatever name you want by editing /etc/lightdm/lightdm.conf adding :

greeter-hide-users=true

This is particularly useful if you use NIS or LDAP services.

For Ubuntu 16.04 configuration for greeters is taken from the configuration files in the following hierarchy:

1. /usr/share/lightdm/lightdm.conf.d/*.conf
2. /etc/lightdm/lightdm.conf.d/*.conf
3. /etc/lightdm/lightdm.conf

The place to add your configuration changes is the second in the list, provided that there is nothing in the last list file to override the values you wish to change.

So, creating a file /etc/lightdm/lightdm.conf.d/50-myconfig.conf and putting in it

[Seat:*]
greeter-hide-users=false
greeter-show-manual-login=true
allow-guest=false

• First entry hides the list of login users
• Second entry allows manual user login
• Third entry disables the guest login account

NOTE: For Ubuntu versions prior to 15.10 replace [Seat:*] with [SeatDefaults]

When changes have been made, restart the lightdm service to have the changes applied.

sudo service lightdm restart

or

sudo systemctl restart lightdm.service

For more information see: Ubuntu LightDM Wiki