Some stupid set of circumstances results in you being cursed. From that point on, you're allowed to play anything on a guitar when you're by yourself or with just one other person (this is the save your partner's or best friend's sanity clause), but you're only allowed to play a single song forever, the same song, when you're with more than one person.
Which song do you choose? (It can be one you haven't learned or mastered yet)
Why did you choose it?
How long do you think it will take you to get sick of it?
Fine print - if you break the rules all of your guitars will instantly disintegrate into piles of ash and metal, so you'd better choose carefully or learn how to play the piano.

Who wants to play?