Navigate to 
Global Data → Appointment Setting section and follow the steps:
Assigning setter to a single lead
Locate the lead you would like to assign ownership to
Navigate to Current Setter Owner column
Select Setter/User from the drop down menu, and it's done
Assigning in-bulk
In case you need to assign more than one lead to a setter, it's possible by assigning in-bulk, click on "unassigned" next to "add filter" to display only contacts without a setter owner and follow next steps:
Click on the check box next to the contact you would like to assign or in bulk
(for selecting all contacts in this view click on "Bulk select all" in the sticky bar below)
Click on Assign leads located in the bottom right of the page
Select the users you'd like to assign the contact to
(you can select one or multiple)
Assign for → the next 24hours, 7 days or unlimited
(Duration for how long you'd like to have Setter assigned to the selected contacts)
Important notes
In case you select contacts that already have Appointment Setting outcome, you will not be allowed to assign such leads to Setters.
Assign leads button on the bar bellow will be grayed out and show error message as per screenshot below.
You'd need to deselect leads with Appointment Setting outcomes before being able to proceed, or alternatively, you'd deselect all, turn on the filter to show all Unassigned leads with no outcomes.