It's currently still in the pilot stage, I believe. You can sign up for the AGB program at FRA; they have an office right next to the Lufthansa first class check-in. So far, it's free. You fill out a form, give them your passport, and they run a background check then take an iris scan. You can then use the special gates to clear immigration when you arrive in FRA (quick iris scan/passport scan and you're on your way).

While completing this registration in FRA, you can fill out an additional form for Global Entry. They'll then give you a promotion code for the program. You then have to complete the online global entry application and pay the $100 application fee. They compare your data with the German program and if you are conditionally approved, you can schedule a visit at a US Global Entry center. You have to schedule a visit within 30 days of receiving your conditional acceptance, however, you don't have to visit within this time. I've also heard that they'll accommodate walk-ins when you're living abroad and only in the US for a short time, but it pays to call the center and ask.