Each event can specify a particular person to check in volunteers as they arrive.  This is setup on each event under the Check-Ins section.  Make sure the check-in type is Administrator will check in volunteers.  If that is checked, you can specify a person 'Who will check them in?'.  This person will receive an email the day prior to the event.  The email will contain a link which will show them all users who are signed up.  Next to each user is their role, time and a button to check in/out.  The person you specify to check them in does not need administrator access, they will be granted access to manage the sign ups for this particular event.