At present, Trojitá does not have a graphical addressbook, though one is actively being developed. However, in the meantime, there's a very effective work-around. Upon start up, Trojitá reads the address book located in ~/.abook/, which you can create using the console-only program called abook. Running abook in a terminal window like Konsole, you can add, delete, and edit address book entries. Trojitá doesn't manipulate that address book in any way, so for example, it won't add new entries as you write messages. But if you've got an .abook address book, Trojitá will auto-complete entries using it.