How do I assign Tester to a Test Run?

To assign the Tester to Test Run, Open Test Run > Click on 'Execute' 

Step 1: Expand 'Bulk Actions' menu

Step 2: Click on 'Assign Tester'

Step 3: Select 'Test Case/s' using the checkbox

Step 4:  Type Tester's name in 'Assign selected To' and select suggested Tester's name to assign the Tester.

Here is the link to help documentation.