On this moment I can select only one group for torrent. I can add this torrent to other groups later but on "Add torrent" screen I can select only one group.
Because groups in kTorrent works like tags and same torrent can exist in more then one group I propose to improve interface for groups.
1. Add ability to select more then one group, look how this function created in FireFox 3.0 (click on star in the address bar).
2. Add ability to create new groups from add-torrent window. Let user type any new ta... groups or select them from list.
3. Add in configuration ability to specify default groups for new torrents.
4. Change "Add to group" function in context menu to add ability to add torrent in more then one group at once (new position in submenu, "Add to groups...").

Look at screenshot with tags interface in Fx3.0: