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.
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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
/gquit
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.
To leave a guild, you click on the guild tab in the social menu. You then just right-click on your name and select 'Leave Guild'. A message will then come up saying 'Do you really want to leave this guild?' and then select 'yes'. Using the chat command '/gquit' will also work.
As you enter the main gate, hang a left and run up the long curvy hill. It is a little way up.