Final Fantasy XI features a somewhat confusing server balancing system, centered around codes called "worldpasses." While the system definitely works, you're going to need some tips to get the most out of it.
When you start a new game, you get permanently assigned to a server - at random. In order to be sure to land on the same server as your friends, which is the only way to play together, one friend needs to get set up first. Once on a server, he can buy a "worldpass" to invite more friends to join him on the same server. Worldpasses are inexpensive but aren't free, so the first person to start may need to acquire some money before he can invite anyone over.
There are two kinds of worldpasses, normal and gold. Normal worldpasses are usually cheaper, but can be used five times before they expire. There are no benefits to using a normal worldpass besides saving your in-game money.
Gold worldpasses have a low, fixed cost and have a number of benefits that come with them, most notably a slew of free items. Gold worldpasses can only be used once and cost a little more, but many will find the additional items worthwhile.
Once you've purchased your worldpass, you have one week before it expires. Simply give the code number and your server name to your friends, and have them input those into the game when asked. Soon, you'll be adventuring together.