Hi,
I'm sure you wish your friends well. Wouldn't they rather have a bassist in the band who shares their musical tastes? Wouldn't they rather have someone in the band who enjoys the music they want to play? If they have hijacked the band and insist on taking over the repertoire, they really haven't left you much choice. If money was involved, if they were paying you to be a hired gun/sideman, then I would stick around for as long as I was obligated, collect my pay, and then give notice. But this doesn't sound like that kind of situation. If this is just a casual throw-together band for a party then I wouldn't worry. If you are going to get grief from your co-workers, just be up front with them. It's better you left sooner than later. It would have been much more awkward later after they started gigging, if they ever start gigging. I don't know what the music scene is like there or how prepared your ensemble will be to put on a show.
I hope the situation doesn't cause any undue stress at work. Just tell everybody what happened. You started the group to do one thing. The group was hijacked and started doing something else that you were not on board with. Hopefully people will understand. I wish you all the best.
Thank you for your indulgence,
BassCliff