Method for operating an access control system for controlling access to an access-restricted zone in a building or a site, wherein the access control system comprises a transmitting and receiving device, a memory device (26, 28), a processor and an image processing device (16, 22, 24), wherein the method comprises a registration process for a visitor, wherein, during the registration process: invitation data generated by an electronic host system and transmitted as an electrical signal by the host system are received by the access control system, wherein the invitation data comprise an identification number of an invitation and an appointed time when a host expects a visitor in the access-restricted zone; a visitor profile assigned to the invitation is created in the memory device (26, 28), and the invitation data are stored in the visitor profile, wherein the memory device (26, 28) contains a database which is provided for storing user profiles of access-authorized users and visitors; image data of the visitor, the identification number of the invitation and a device-specific identifier of the electronic device are transmitted by an electronic device of the visitor as a further electrical signal via a communication channel, specified in the invitation, for storage in the visitor profile created for the visitor; the image data of the visitor, the identification number of the invitation, and the device-specific identifier of the electronic device of the visitor are received by the access control system; the image data and the identifier are stored in the memory device (26, 28), wherein the image data and the identifier are assigned to the visitor profile created for the visitor by means of the identification number of the invitation; the image data are processed by the access control system to generate a reference template; and the reference template is stored in the visitor profile. invitation data generated by an electronic host system and transmitted as an electrical signal by the host system are received by the access control system, wherein the invitation data comprise an identification number of an invitation and an appointed time when a host expects a visitor in the access-restricted zone; a visitor profile assigned to the invitation is created in the memory device (26, 28), and the invitation data are stored in the visitor profile, wherein the memory device (26, 28) contains a database which is provided for storing user profiles of access-authorized users and visitors; image data of the visitor, the identification number of the invitation and a device-specific identifier of the electronic device are transmitted by an electronic device of the visitor as a further electrical signal via a communication channel, specified in the invitation, for storage in the visitor profile created for the visitor; the image data of the visitor, the identification number of the invitation, and the device-specific identifier of the electronic device of the visitor are received by the access control system; the image data and the identifier are stored in the memory device (26, 28), wherein the image data and the identifier are assigned to the visitor profile created for the visitor by means of the identification number of the invitation; the image data are processed by the access control system to generate a reference template; and the reference template is stored in the visitor profile.