Blizzard has instituted a spawning system in StarCraft 2, granting free access to gameplay features when moochers party up with players who have purchased the game.
When you jump into multiplayer, you play the most current version of the game owned by at least one person in the party. If you have the Starter Edition and you're playing with someone who has purchased StarCraft 2: Heart of the Swarm, for instance, you'll both play Heart of the Swarm. The same goes for parties greater than two.
Spawning is being rolled out globally today. Not all regions have access to it yet, but Blizzard assures us it's in progress.