If you are trying to add a team member to your account and getting this error message: "This email already in use!", it means that this user already has a TestProject account.
In such a case, you have 3 options:
Ask your team member to delete his personal account - If it's possible, your team member can delete his own personal account. By doing so, his email address won't be in use anymore and you could easily add him to your account. If your teammate already have a tests that you would like to keep follow this article to transfer the tests to before deleting the account.
Use a different email address - You can use a different email address to add your team member to your account. This way, that teammate can keep his own account and also work in your account.
Use alias in the email address - If your team member's email provider supports alias, you can add him to your account with alias email address. For example, I can add Karen, my colleague from the above screenshot, to my account like this: firstname.lastname@example.org
You can write anything between the "+" and the "@" signs.