Blizzard has introduced a system called spawning into Starcraft II, allowing its Starter Edition and Wings of Liberty players to be temporarily upgraded to Heart of the Swarm accounts when playing with friends.
When joining a lobby with a friend, the game will set itself to that of the player with the most Starcraft II content. If a Starter Edition player--Blizzard's free-to-play version of Starcraft II--joins a friend who only owns Wings of Liberty, for instance, then the two players will play at the latter level, but if the group is joined by somebody who owns the recently released expansion Heart of the Swarm, then all three players will be granted access to the expansion's content.
Using spawning, Starcraft II players will have access to all of the game's team-based ladders (two versus two up to four versus four) alongside custom game modes and the Starcraft II Arcade. Team-versus-AI modes are also available.
Players are not able to access the single-player campaign using this method, nor will they be able to play in Starcraft II's one-versus-one modes. Starter Edition players will only be able to play as one of Starcraft II's three races, which at the time of writing is Terran.
Additionally, Blizzard says that players who use spawning will get an in-game achievement and portrait.
For more on spawning in Starcraft II, check out GameSpot's interview with Blizzard production director Chris Sigaty
Starcraft II: Heart of the Swarm was released earlier this year, and requires a copy of Starcraft II: Wings of Liberty to play. Blizzard reported that the expansion sold 1.1 million copies in its first 48 hours on sale.
For more information on Starcraft II: Heart of the Swarm, check out GameSpot's previous coverage.