Step 1: Navigate to Jobs and select All Jobs
Make sure you are in Grid View instead of List View if you wish to multi-select your jobs.
Step 2: Tick which Jobs you wish to assign to user and select Assign To
You can also tick the box at the top of the column to select all jobs.
Step 3: Select User
Step 4: Confirm Assignment
As you can see below, that user is now assigned to the selected jobs.