Purchase Arma 3 Alpha now and receive the following:
Alpha-access – Be one of the first to play Arma 3 and contribute to its development by providing feedback. 3 Alpha Lite Invites (not yet available) – Introduce your friends to Arma 3 by inviting them to the free invite-only Arma 3 Alpha Lite (available from March 14, 2013 until June 18, 2013). Beta-access & Full Game – Gain access to the Arma 3 Beta as soon as it becomes available. Get a Steam version of the complete game upon release.
-Pasted from the Steam page
I would assume at least before this date, but this date is for the guest pass, so it could be anytime really.