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You might have to sign up for those forums but its still worth getting the beta.
If you do not feel like signing up then here is the tutorial below!

Step 1: Proceed To this Website
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Step 2: Sign up by clicking the Green Sign up button below is the picture to it.
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Step 3: Finish signing up and then click the download button in the top right corner

Step 4: Download the desktop app

Step 5: When finished installing deskstop app Sign in to your Raptr account

Step 6: Enter your windows Live ID and Password/ below is a example.
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Step 7: Let it "Analyze Games" and all other information

Step 8: Skip everything until this page/example below.
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Step 9: Click the little Xbox Controller in the corner. Make sure it says "Name of game" and "Number of games" added to your library and followed. Also wait until a little box in the corner says it has recognized all your achievements.

Step 10: Wait for it to say, "You are now eligible for a reward" Personally I have no idea how long it takes, due to me leaving my computer and not comming back for several hours so i recommend you not wait.

Step 11: While on the home page. Click Rewards Located /below is another example pic
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Step 12: Under Eligible: Click Claim for Xbox 360 Ghost Recon: Future Soldier Beta

Step 13: Click Claimed Rewards and Click View Reward
A box like this should come up and your code should be there /example below.
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Hope this works for everyone worked for me.
Also This site says it has alot of codes dont know of its true but seems legit.
Works For Ps3 Too
Hey Guys 1 more thing i don't know if it works to get more than 1 code for the same account but i guess you guys can try.