How can a user id be linked to an employee?

Make sure you have created both an employee and a user. Now you are ready to associate them with one another. Go to the Administration - User form and select the desired user. Now press the button User relations. Ax will automatically create a new record, if no link already exists.

Go to the second tab page and under Employee, select the wanted Employee.
You have now successfully linked your Dynamics Ax user to an employee.

Linking a Ax user can either be internal (employee) or external (customer, vendor, business relation).
Same method applies for linking with external contacts.