To get a usertag (badge) for a specific guild, you must go to the "Group Memberships" area located in your User CP - of course, I'm a good person, so the link for it is right
here.
Once you go there, scroll down to where it says "Shadow Sorcerors," fill in the little bubble in the column to the right, and hit "Join Group." Davy should then get a message, and once he approves you in the group, you can get a nice, shiny tag
Happy pirating!
