A clan is a group of members with at least one leader that trains players, runs clan events and wars other clans in minis, preps and more.
There is a section where all clans unite together. The Gathering of Champions( Dead)
Isgood currently runs a thread on the SP Forums that lists all guilds: Guilds/Clans Thread
Sons Of Saints - Clan Website
Raging Infernos - Clan Website
Empire's Throne - Clan Website
Elementals Reborn - Clan Website