I suppose it depends on your definitions of reason and plan. You don't need to have something very formal, but you need to be able to answer the question "what are we doing?". That way, they can decide if they want to be part of it. You also don't want obvious boredom, where everyone's sitting around with nothing to do.
Your "plan" can be as simple as inviting the guys over to hang out and drink beer. You let the night go wherever it does. If everyone wants to play a board game, or go to the bar, or get Taco Bell, or go to a strip club, just let it happen. And if everyone is happy hanging out and drinking beer, that's great too.