To assign a workflow to a user follow these steps:

  1. Go to the User's record - In the top menu People>Everyone, select the user's profile you want to assign a workflow.
  2. Go to the Inbox tab, and select sub-tab "Workflow".
  3. Click on "Assign Workflow" button, and select from the dropdown menu the workflow to be assigned. 

Learn more on "How to create a workflow"