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.