If you want to join an iOS beta program, the developer will need to know the unique ID of your iOS device – the UDID. This is because beta apps are distributed to a limited set of devices, not through the App Store and the developer has to create a development profile that includes the UDIDs of every device permitted to install the test app.
To find this ID, connect your device to iTunes either wirelessly or via USB. Select the device in the side-bar, then select Summary in the tabs at the top. You will see a display similar to this: