Not much. Just ask for an invite to a casual guild, open the guild panel, represent that guild (required to get the buff), port to the guild hall, go into the tavern, and turn on the 10% EXP buff. Port out. Open guild panel, turn off representing that guild, and then forget all about guilds for the rest of your life.
The name "Guild Wars" is a holdover from an early iteration of GW1. That game was supposed to be entirely PvP focused, with the PvE stuff being a tutorial. During development, more and more focus was placed on the PvE side and the PvP thing became more of a side mode. Eventually the name "Guild Wars" just refers to a historical event that takes place in the backstory of GW1, and has no bearing on the plot or anything going on in the game you actually play.