An email can only be linked to one Printful account (Source https://help.printful.com/hc/en-us/articles/360014067799-Can-I-add-users-to-my-Printful-account). So new users can only join the MUI org after being invited to their email.
If they have already created an account, it will fail with “A user with this email is already registered!”: they can:
Remove the user with the delete button

Click Remove button to confirm deletion.