Which is correct?
a. On behalf of John and I, we would like to thank you.
b. On behalf of John and me, we would like to thank you.
Firstly, I think you should be using 'I'not 'we' as you alone are actually doing the thanking. As it comes after a preposition, 'me'would be better than 'I', but 'on behalf of me and John' sounds a bit strange and it's very common to say 'on behalf of myself and John, I...''