Once you've added a Manager to a course or a group, ensure that they have the Update Course Progresses & Grade Assessments permission enabled.
Course Managers will be able to manage ILTs for everyone in their assigned courses, while Group Managers will be able to edit the ILTs for the users in their group that are also enrolled into the course via the group.
When they log in, they will need to click on My ILTs in the Manager section.
From there, they will need to use the drop-down menu in the upper-left to select the ILT they want to manage. By clicking on the Capacity icon, they will be able to register users to the timeslot (Group Managers will only be able to register users who are in their group), as well as mark the attendance of users (Group managers will only be able to mark the attendance of users in their group).