Cant add my acccountant as a team member on multiple quickfile accounts?

sorry thats not what i mean

quickfile account 1
my accountant is setup as a team member with his email address (not gmail).

quickfile account 2
my accountant is setup as a team member with his email address (not gmail).

quickfile account 3 and 4
i cant add him as a team member?
This Email address has already been registered to another QuickFile account. To manage multiple QuickFile accounts from a single email login please register to use Affinity.

why can he be a member of 2 of my accounts but not 4?
surely this is silly, he wont be able to log into any of his other clients accounts with his email?