After you have clicked the button, you will be asked to login to your account.  You will see a screen such as this:



Note if you look under settings then Plans and billing, you will see that it indicates you have a free account as shown here:

This is normal.  The account is free, but the workspace you have joined by invitation enables the features you will use.

The workspace is a premium account. Therefore, you are able to access premium features under that workspace without the need to have your own premium account.

 You are now ready to use Vectary in class.