I'm assuming you are using FMP 11. If so, you will probably want to put the referrers in a separate table. Then you could use a type ahead script and script trigger to filter them by last name in a portal and then once the right doctor has been found a button in the portal row with a script to set that doctors ID into the patient's table. A type ahead has for the user to enter characters into a filter field that populates the portal with every name (I would just use the last name) that meets the filters criteria, S brings up every name that begins with S, SM, SMI, etc, until you spot the name you want. If you complete the name and have no hits left, then another button to create a new physician record. (set up your tables so that each record has a unique ID field and use a relationship to populate the referrer field in the patient's record)
In my experience validating names is actually more problematic, most often a separate calculation field would be created and a simple formula truncating the lastname&firstname&etc. and validate it to be unique. Then you get the father son clinicians who take you to the next level. Not to mention the patients who can't even remember the name of their primary care doctor. Also be prepared for newpaper, yellow pages, friend, neighbor or current patient, etc.
Adding a portal for patients of the same doctor is easy because relationships are bidirectional so in the doctors layout set your portal to filter off of the doctor's ID.
Bumper, Thank you for your response! I really appreciate the information and that gives me a nice framework from which to work. I'm about 25% of the way through the missing manual, so I'm hopefully gaining the skills to implement your suggestions. You were correct that I am working with FM11 Pro, sorry I didn't state that in the original question. Anyway, thanks again. I'll post back when I either have a mostly finished project or run into stumbling blocks that I can't overcome.