Your rule should first use condition "Entity has field" on your entity reference field from the form -> the field where user selects businesses. In this case your form submission is the primary entity that your rule will target.
And then you'll have to loop through selected businesses and extract business email from each business that was selected by user in that form submission.
You can use components for getting email field out of your business entity or you can install "Conditional Rules" module (CR), which will allow you to write all the logic in rule's "actions" section without using components.
So your rule (if you installed CR) would be as follows:
Events: After saving a new entityform submission
Conditions: 1) Entity has field [entityform:field-select-business]
Actions: Your actions will look something like this.
Loop: here you loop through your Entity Reference field - [entityform:field-select-business]. You cannot loop through your Email field directly - every email is taken from refernced entity.
IF - here you will select conditional "Entity has field" Entity: [list-item], Field: [field_business_email] <- your email field. This brings email field into the rule's scope, otherwise your rule can't see the email field. This part is not possible to achieve in Actions section of a rule without "Conditional Rules" module.
And in the end - nest "Send Email" action under IF condition and you should be good to go.