Hey Thomas B. ๐ Thanks for trying out Arize! This is actually the support channel for arize-phoenix, the OSS platform so you might have better luck in Arize AX Support going forward ๐
I used to work on the platform though so I know the answer to this one - the current platform only lets you belong to one account so my guess is that the user you are trying to add to an account already has an account and belongs to that.
It's a super compelling use-case! I'd definitely reach out to support in the other slack channel and a support person will get back to ya!