Blizzard has launched the “Spawning” feature for Starcraft 2. Spawning, back in the days of Warcraft 2 and the original Starcraft, allows a player to use one game disc to spawn in a second player who doesn’t own the game. In Starcraft 2, players who don’t yet own the game can get the Starter Edition and still be able to join games with players who own later content or expansions.
“Any player at a lower expansion level who joins a party that includes a player at a higher expansion level is Spawned up, and gets to play using the features and options of the highest expansion level for as long as they’re in a party together.” Blizzard said on Battle.net.
“For example, when a Starter Edition player joins a party with a Wings of Liberty–level player, both are instantly playing Wings of Liberty. When those two players then join a party with a Heart of the Swarm-level player, they all immediately gain access to Heart of the Swarm content, and they keep that access until their party no longer includes any higher-expansion level players.”
Blizzard has also released a video explaining the Spawning feature which you can watch below: