Easiest way is to schedule the event on the in-game calendar. It will let you invite your whole guild or just select levels/classes.
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Ask for an invite to A guild. Only Officers, Leaders, or The Founder of a Guild can Invite You. You can also make your own guild
If you are high enough in the guild that you have this power you press hot key "o" and select guild along the bottom. Then at the bottom there should be a button that says "invite guild member" click it and type in the name of the person you want to invite.
you have to be in a guild to do guild battles and you have to be the leader to do a battle
To level up your guild you have to put Quest Items into your Guild Warehouse.
You have to be a member, its by the fishing guild.