CW Flyff is full of small communities, called Guilds. In addition to being social groups where you can meet new people, Guilds can also work together in PvP activities such as Siege and Clash, or in PvE activities like Asmodan's Trials.
To find a Guild, you can either use Guild Finder, check the guild-recruitment channel in Discord, or just meet players in-game and ask to join their Guild (though this works best with friends, and not strangers).
You can also make your own Guild by pressing "G" (default hotkey) and clicking "Create". From there, you can change the name of your Guild, and begin recruiting members through Guild Finder or by inviting them manually.
Guild members receive buffs depending on their members, which you can see at the top of your screen. To level up your Guild, you can donate penya and quest items at the Public Office NPCs in Channel 1.
In addition, Guilds may also receive special buffs for participating in PvE content like Asmodan's Trials, or PvP content such as Clash.