1: Click on your user CP
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2: Hit "Change Gamertag"
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3: Enter and save your Gamertag
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Then wait till the next update window for it to appear
Note: Due to code limitation, tags update once a day.
Steps to add people from website
1a: Click someones name either in Member Block on the bottom of the main page
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1b: Or click the Gamertag on their post headers
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2: Login if prompted and click Add Friend
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Note: This shows "Remove Friend" because Dern is already on my list.