Well, you can play single player and pass the guitar to each person in the room. If you get boo'd off stage you have to finish your entire drink. If you finish the song, the amount of percentage you are away from 100%, you have to take that many drinks. Also, to even the playing field, If you finish a song with 100% you and one other person of your choosing have to waterfall for ten seconds.
If you have two guitars, the head to head rules apply. You have to drink the difference between your % of notes hit and the person who beat you. If you get boo'd off you have to finish your drink. So much fun.