You can assign a user to an existing loan by doing the following:

  1. Within the loan, go to the Setup Options screen.
  2. Update the drop down for the role you wish to update, and select the user you wish to assign.

See these steps below:

Note: If you are a loan officer, you can set a team so your processor and loan officer assistant are assigned automatically! See how here:  Setting a Team to Automatically Assign Processors and LOAs