You could have a second access code included in the invite but not printed right on the window in screenshots.

It would be similar to how a credit card number and CCV code are functionally the same as one longer number, except that you don’t go writing the CCV code alongside the credit card number, and that keeps it more secret.

Still not as frictionless as “anyone with the number can join,” but if this continues to be a problem it might be worth doing.