Blizzard has reintroduced a very old friend back into the Starcraft fold, a process known as 'Spawning'. What this was is players were able to boot up a copy of Starcraft 1 on their PCs after their friend had on their PC with the same CD - hence giving a 'free' version of the game to their friend while they play.
Just announced Blizzard has brought back this process to Starcraft 2 in the form of allowing members of the party to freely 'upgrade' to the highest level expansion that a member of the party currently has. For example a player with the Starter Edition (found free on Battle.net itself) can party with a player with the current Zerg expansion Heart of the Swarm, both players then are classed as 'owning' HotS as long as they remained partied.
Doing this will unlock Multiplayer (ranked and unranked), all custom game maps, Vs. AI and the Arcade in the highest level of content the party members have, those with the Starter Edition will only be able to play the race currently playable in that version (at time of writing that is Terrain), and have limited access to social features, for example Clans are locked out to those not currently owning HotS. However the limit of players allowed to do this is the current party cap, thus 14 of your friends can benefit from your purchase.
To celebrate this new feature, Blizzard have added a new Achievement and Portrait to the game for players Spawning in new players or are the ones being Spawned at this current time. The release date of this is right now, however globally it may take until the end of the day to become active in your area.
For the full information watch the following Youtube video below the divide.