Table Design Of User Table.

We are in the early stages of developing a small project and I have a question I would like an opinion on.

When deciding how to design a table to store information on users who will be different types.

The project involves setting up webpage that allow " appointment setters" to book demos for our Sales reps. We want to keep track of how many each appointment setter books.

I  have a user table with user_id, fname,lname, type. Where type represents the type of users they will be either "appointment setter" or "Sales Rep".

In the Appointment table I would like to store the Appoinment setter ID, sales rep Id, date/time appointment was booked and some other data on the appointment.

My question is how do I set up the relationship from  the appointment table back to the user table.

Since each record in the appointment table will have a App Setter ID and a Sales Rep ID.

Should I just pick either the app setter id or the sales rep id as the foreign key back to the userID in the user table. Or is there another method how to handle this.

Any thoughts or articles would be greatly appreciated.

thanks,