Each iPhone or iPod Touch has a Unique Device Identifier (UDID), which is a sequence of 40 letters and numbers that is specific to your device. It's like a serial number but much harder to guess. It will look something like this: 2b6f0cc904d137be2e1730235f5664094b831186.
Your iPhone can only install programs that are approved by Apple. Applications in the App Store have been approved by Apple for general distribution, but special beta customers get to try the app before it's in the store. We register your UDID with Apple so they can approve our application specifically for your iPhone.
You can:
Install and run Ad Hoc Helper. Click here
It will create an email with your UDID.
Send it to yourself and then copy the UDID from your mail and paste into the required field for UDID on the sign up page for becoming a Squace VIP iPhone Beta tester.