Blizzard have gone and done a nice: the developer has activated a feature which allows all players to play together no matter what version of Starcraft 2 they are running, even if it’s the starter edition.
They call the process ‘Spawning’ and the way it works is that if you are in a party with a person running a more modern release of Starcraft 2 than yourself then you temporarily gain access to all that exclusive content.
In theory, what this means is that if I were to be playing a starter edition version of Starcraft 2 and I joined your party in a multiplayer lobby, and you’re running a copy of Heart of the Swarm, then Blizzard would allow me to play on all the maps that came with Heart of the Swarm and with all the new units. As soon as you leave the lobby then I lose access to that content.
It’s a wonderful feature meaning that you can play with your friends without anyone having to limit the maps they’re allowed to play on.
The new spawning feature is outlined in this Blizzard blog post and this video runs you through the basics of how it all works: