An email can only be used once in Tailwind, thus if someone is already an owner of a Tailwind account and you try to add their email address as a Collaborator you'll likely see this error message:

To fix this, try altering the address just a bit and add +tailwind after the name in the email and before @domain.com

ex: [email protected]

Within Tailwind, this is treated as a unique email address. However, email clients will ignore the "+" sign and all text following it, meaning you'll still get email alerts from Tailwind in your standard email inbox. You can enter any text you want following the "+" sign -- it doesn't need to be "tailwind."

Finally, the collaborator will receive the email invitation as normal, but moving forward they will need to use this altered email address in order to log into your account instead of their own.