To make things more confusing, responding to an invitation to drinks with "k!" might be ok, depending on who's asking and the context.
If it's your roommate asking, and they're doing it because there's a sporting event or because drinks are an established ritual, it'd be fine. If it's your boss, or a new (potential) friend or colleague, a possible romantic interest, or a close friend and it's not something they usually do or invite you out for, then those situations have a lot more weight, and expect a more fulsome and engaged reply.