Presuming you have a column type as Person or people in your Document Library, List, or Task (or whichever app), if you are working on SharePoint Designer 2013 (you can download it), you should be able to create a workflow and in it, once you set up the IF conditional, for the "then" condition, go to the "Action" menu and from drop down actions:
Step 1: Select Action "Send Email"
Step 2: click to edit the 'users' and click on the book icon of "To" field
Step 3: Choose "Workflow Lookup for a User..." and click on ADD
Step 4 and 5: From the "Lookup for Person or Group" window, choose the correct source in the source "Field from source"; in my case, the column titled "Dept. Stakeholder" Step 5: Choose "Return field as" field as 'Display Name' and click ok
End result should look like this:
The above steps created the correct algorithm engine to achieve my goal. In testing phase, I switched users for an assignment multiple times in the Column value in the Document Library. As long as the primary parameters to trigger the workflow met, SP activated the workflow and send out the email.