Well, you can organize PvE events on your Guild Chat, like Sorrow's Furnace, yada yada. You can also organize Guild vs. Guild combat against another guild. You need to have at least 4 human players, and you end up with an 8-man team (there are henchmen available). If you want to get into GvG, you'd be wise to observe the top games (not sure if seasonal stuff is still in observer mode) and watch the strategy, and try to replicate it yourself.