Each "entity" in an Entity Relationship diagram need not refer to a different FileMaker Table, but could represent different occurrences of the same table in the FileMaker Relationships tab.
Your entire list would seem to have a set of fields in common: Name, Address and other contact info.
I'd set up a table of contacts where you have one record for each individual, whether they be a referral, a vendor, Employee, SubContractor... or some combination of all of the above.
You can the, for just one example, set up a table of referrals where a record in this table use a ContactID field to refer to the name and contact information of the contact making the referral and a second ID field can link to the contact being referred.
Lets See if I understand...
Create a People Table that holds all Fields specific to that person (Name, Home Address, Home Telephone, Credit Card Number, email address, etc.)
Create a Company Table that holds all Fields specific to that company (Name, Company Address, Company Telephone, website address, etc.)
Link People and Companies Together using Contact ID's so that I can know who works where and send letters to them or know their company telephone number using a portal
Use Join Tables by:
Create Join Tables for Referral Office, Insurance Company, and Adjusting Company that only link to Companies in the Company Table
Create Join Tables for Employee, Referrer, and Insurance Adjuster that link only to People.
Create Join Tables for Referrer, Client, Vendor, and Subcontractor that link to either People or to Companies
(I'm having trouble picturing how this would work)
Create links from the Job Entity to individually include Client, Insurance Company, Insurance Adjuster(s), Vendor(s), Subcontractor(s), etc
(I'm unsure how to create fields that would link to either People or Companies)
Both Companies and People can be "contacts". Note the similar contact fields that you listed for each.
Do I use Join Tables or Do I use Links to create relationships?
With both Individuals and Companies merged into one Contacts Entity the number of fields would be very unwieldy correct? Imagine individuals home address, company physical mailing and shipping address, credit cards for both company and individuals.
I cant think how I would create a relationship from one contact to another in one Contact List (ex. Employee / Employer Relationship)
If I wanted to enter a Client that would either be an induvidual or a company the fields would be different (First and Last Name vs Company Name)
Just found Self Joining Relationships which helps clear up the Contact Relationship Issue.
How Can I enter type specific data for either a company or an individual on the same data entry screen?
If you use the same table for both, this should function for you without any major issues, but I would only put contact information in the contacts table. If there is additional data to be recorded for companies, I'd add a companies table and link it to contacts via contactID.
And you may want to experiment with a tab control where you have a different tab panel for each general type of contact--that way you can modify the basic layout to better suit, say a "people" contact on on tab panel and a "company" contact on another.