Whether there are creative differences and tension among musicians, or it's just time to move on from a group, this letter is intended for someone who has to quit being a member of a band.
Dear Band Members,
I'm quitting the band.
It's terrible that it's come to this, but I just can't take the (infighting/creative differences/partying) anymore.
We made some great music, and I am grateful for the opportunity to collaborate with such a gifted group of artists. I consider you my friends.
I can play the gigs we already have lined up, but that's it. I'd suggest that you advertise for a new (drummer/lead singer/bass player/etc.) as soon as possible.