If you have been part of a business that has an account on Bookem, you won't be able to create a new one by signing up again with your existing email address because you don't need to: you are already registered as a user on Bookem.
Follow these steps:
Log in with your existing account. If you can't remember your password, choose Forgot your password and follow the instructions to reset it.
Once you're logged in, open the calendar, click the plus sign in the top right corner. Then, choose "New business."
Add the details of your new business in the modal that pops up:
the name of your business, and
the 'handle'/subdomain for it, this will be used for your public booking url, and the address your Bookem emails are sent from.
In the example below, emails from this business will come from firstname.lastname@example.org and clients will be able to book online at acmedocs.bookem.com.