The file should use a standard format. I'm considering using vCard. This would allow configuration of the groups with any address book application that supports vCard.
Update: I've decided not to use vCard. vCard does not really seem to have proper group support and/or the KDE libraries/Akonadi have proprietary group support.
Instead I'm using an ini-style config file via KConfig. Apart from being a simple format that does not require new dependencies in libkleo, this has the advantage that KConfig's ability to lock down configuration options can be used to create immutable groups.