The simplest way to handle this in Fergus is to create a customer for the central company (e.g. the real estate firm), set a main contact, and add the other points of contact as Other Contacts:
When emailing one of the contacts, start typing and you will see suggestions if you've entered email addresses for your contacts.
You can also change the main contact for a specific job to one of these other contacts by clicking the x icon beside them in the job card > Job Summary:
You may need to change the name of the customer on the invoice. You can do this on a draft by clicking on the address and typing the correct details:
Note that the invoice will still sync to your accounting platform under the customer the job was created for.