Your relationships as I understand them:
Projects::ProjectNumber = Join::ProjectNumber
If you place a portal to Join on your Projects layout, with "allow creation of records..." enabled for Join in the Projects to Join relationship, you can place Join::EmployeeID# in your portal row formatted with a drop down list or pop up menu of ID#'s and employee names. You can then place the name fields from Personnel in the same portal row. When you select a person by selecting or entering their employeeID#, this creates a new record in Join and the value of Projects::PRojectNumer is copied automatically into ProjectNumber of the new Join record. And you'll find that the name fields or any other fields from Personnel that you choose to add to the portal row will display data from the selected personnel record.
Thanks for the help! It worked.