In WoW you can add their battle-tag ID (Same as our SE ID) into your friend list.
So every time they login with their character you will know and can invite them to your party for the Raid and Dungeons.
Some of player meeting through forum and form a static just like that.
Normally all other party member will be go into Party leader world instance.

It's same like having a friends from another dimension and invite them to have fun in your world