Playing multiplayer with other people (whether friends or strangers) is a great way to enhance the game or make it more challenging.
Types of Multiplayer
In the Multiplayer menu, this is the default. CKII will scan for player-hosted games on the Local Area Network. The computers of the players must be physically connected somehow, or a simulated LAN can be established with a program such as Tunngle, Evolve or Hamachi.
In the Multiplayer menu, this is called 'Join Internet Game' and requires you to input the IP Address of the host.
The Metaserver is run by Paradox and is a good, stable host for games. It serves as a public meeting ground for people who want to play online, so someone can play multiplayer without other friends. To use it, players must create an account and register their game at Paradox Plaza, then log in with that account to access the Metaserver.
When the players first join the game, they download the save (if there is one) from the host. Then they choose their characters and signal readiness. The host server performs another check when all players enter the game after the host has begun. Thereafter, the host controls the speed (though any player may pause the game by clicking the date. Note that the space bar will not pause in Multiplayer). If the speed the host has selected is too fast for players, the host server will automatically lower the speed to allow the slowest player to remain in sync.
The game will automatically pause if a player disconnects or becomes out of sync. It is advisable to save the game any time either of these happens.
Because checksums change each time a player resigns from a game, it is often the case that all players involved will have to restart the entire CKII game in order to regain the same checksum they were playing on.
There should be no problem if some players in a multiplayer game have Sword of Islam (and other DLCs) and others do not.