How would I add a field on the general tab of a group?
The Tridion 'FBI' project is still under development, it contains some great examples of:
- Extending the GUI
- Using the 'Extension XML'
you can check it out here: https://code.google.com/p/tridion-field-behavior-injection/
I'd recommend to install it, and review:
1) How the extension is 'hooked into the schema and component dialogs' - you'll need to do this in your security dialog
2) How form fields and field groups are injected into the diaglog
3) How this information is stored (extension XML) into the CM database. If it's not possible to use extension XML with non-content items, then here you're best best to to look into custom 'app data'
i hope this helps a little.
Julian Wraith has a post about GUI extensions that might help you further.
I do agree with Johnwinter that additional information would be nice to help you any further.