Blizzard has been allowing players to get a limited StarCraft 2 experience for free for quite some time now with its Starter Edition, but now players who don't want to shell out cash for the full game can get an upgraded experience as long as they have a friend who isn't so stingy with the cash.
Blizzard has re-introduced spawning, which allows players who have a lower level version of the game to be "spawned up" to a higher version of they group up with a person who has the upgraded title. So if you have Wings of Liberty, but group up with someone playing Heart of the Swarm, you will get to play HotS as well as long as you stay grouped, according to a post on the official StarCraft 2 blog. The process is similar to what was used for Warcraft 2 and the original StarCraft to allow players to bring in their friends. If a player has no version of the game at all, they can create a Battle.net account and download the starter edition, then group up.
For those of you who love achievements, Blizzard has added one for spawning up a pal, and you get a new portrait to boot. Check out the video describing process in detail, narrated by community manager Cloaken.