When you create a task, you have the option of assigning it to not only yourself, but to multiple people at the firm. Assigning a task with someone means that the task will show up on their task list and they will be able to mark it as complete.
In the screenshot below, we've highlighted the "Assign Task" column on the right. Simply check the box next to each person that you would like to assign the task to.
You can edit task assignments at any time. Simply open your task list, then click the Edit button (pictured below) for the task. You will be taken to the task editor where you can update task assignments.