The estimators you see listed in the drop down are coming from your company’s contacts.  In order to add them you’ll want to open your company’s business record.  The quickest way to do that is to do a search on the “Other” category. It should only bring up your company.

Here you’ll click on the “New” button and add in the person.

Now when you go to the drop down for Estimator in the Project tab, you should see them included.