By default, Planless.io prediction engine will automatically calculate who, on your team, is the best suited to realize a task.
In the Task detail panel, you will see which teammate is "predicted" to work on it.
But you can always force a team member on a task by clicking on the "Will work on it" section. When doing so and confirming it, the label would change to indicate that the affected team member is "forced".
To remove a forced team member, just use the cross icon next to it.
You can also quickly see who will work on a task in the task list. Next to each task, you'll find the avatar of the affected team member. Pass your mouse over it to see his name!