Within your volunteermatters system each contact record can have an associated site user record.
A site user record is system access for a volunteer. When a contact registers via the registration form an associated Site User Record is created in addition to the contact record. The site user record is the username and password and any additional user roles (administrators and organizer). By default every site user record has the "Volunteer" role.
For those administrative roles:
An administrator can do anything in the system: Contacts, Projects, Credentials, Configuration. The ability to grant administrative rights is available only to administrators in the system.
An organizer has access to the Projects area and they only have access to those projects they have been specifically assigned to by an administrator. They can not create projects on their own. With a recent update to the system, the organizer also now has limited view-only access to the contacts list. The fields available to the organizer can be set by an administrator in the configuration area. So they can make the field view as narrow or detailed as desired.