Those method refer to the existing methods.
Do you have class Data_ContactsGroupCommon anywhere?
At first, when you create your own functions, remember to put those in the files that will not be overwritten during update. In other words, create your own file!
You've got two options.
1. Create your own module with install and common file, where you'll put your callbacks. Set those callbacks in the install file.
2. Use benefits of autoloader, create a file in existing module with static method that you'll set as callback in the admin panel.
First is a harder one, but it's better, because it separates your code from the core. Additionaly you can install such module on the other installation without playing with GUI to set proper callback again.
Please refer our wiki to get more info about autoloader and creating modules.
Regards,
Adam