Resurrecting some old-school sensibilities, Blizzard now allows players to spread the love.
The single-player campaigns in Starcraft are pretty nice, but multiplayer has always been the meat and potatoes of the series. Obviously you need a solid community of players to maintain that kind of atmosphere, so Blizzard's always looking for ways to bring newcomers into the fold. The developer's latest strategy, a system called "Spawning," allows you and your friends to play Starcraft II together with almost all of its online features, as long as one person owns a copy of the game.
The system harkens back to the days of Warcraft II and the original Starcraft, where groups of friends could play together using a single serial key. Spawning achieves the same effect without the need to pass a set of installation discs around the room. Whenever you join a party with another player who has a higher expansion level than you, your game is bumped up to provide access to all the features your friend has - effectively "spawning" you into the expansion. This means you can get a group of friends to install the free Starter Edition of the game, then invite them into a match with your Heart of the Swarm-registered account to grant everyone use of the full game and the expansion's features.
There are a few conditions: you can't piggyback your way into the single-player campaign of an expansion you don't own through spawning, and Starter Edition games that spawn into higher levels are still restricted to one faction. Everything else (additional modes, ranked and unranked team ladders, and custom maps) is at your disposal until you leave the party.