Update: we ran out of keys. Thanks James and Johan for helping out with theirs.
What is ArmA 3 Alpha Lite?
From Bohemia Interactive's website:
Arma 3 Alpha Lite is similar to the regular Arma 3 Alpha in terms of content. However, modding and multiplayer are not supported. People can access Arma 3 Alpha Lite for free, but will require an ‘invite’ to do so. These invites can be obtained via people who have purchased the regular Arma 3 Alpha. They will also be randomly dispersed via our social channels and selected media outlets.