As discussed:

The use case is that you have a project and have several keys both S/MIME and OpenPGP that are related to a project. The Project identifier can be a Mail address or a generic Name.

Kleopatra should offer user interface to Add both S/MIME and OpenPGP keys to a Group.
The Groups should be easily selectable when setting up recipients for file encryption.

The groups can be stored by kleopatra. Ideally the API for them would be in libkleopatra so that they can be easily respected in KMail and Outlook (which use libkleopatra).

Autocompletion for group names should work in the recipient selection dialog.

The Groups should complement tags which use the remark feature on OpenPGP certifications but don't rely on that because we need S/MIME and OpenPGP combinations.