I wouldn't show any anger or frustration to the band but instead, jest about it. It may well be the website hasn't been updated.
Like Lowdown says, they might be wondering why you're looking for another band.
You could say to one of the band "I've got bad news, I've had an email to join a band that fit with my style. Good news is tho, it's this band!", then you can see what sort of response you get & if there's anything that would hint towards looking for another bassist.