StarCraft 2 multiplayer goes free with new 'Spawning' system
Blizzard has implemented a new "Spawning" system in StarCraft 2 giving newcomers free access to gameplay features when mooching off your friends who have already purchased the full game. Spawning operates as sort of an upgrade system. When you jump into multiplayer and are in a party with someone who owns a higher expansion than you, you are instantly upgraded to that expansion, with all of its related features unlocked.
For instance, if you only started playing the Starter Edition and a player joins the party with Wings of Liberty, you will be upgraded to the Wings of Liberty version. If you both are joined by someone who owns Heart of the Swarm, you'll both then gain access to Heart of the Swarm content. You'll retain access to these features until the player with the higher expansion leaves the party.
Spawned players will be able to access the following multiplayer features: all vs AI modes including team games, ranked and unranked team ladder matches, all custom game maps, and complete access to the arcade. For the full rundown, check out Blizzard's video explaining the Spawning system.