1. A document registration apparatus comprising: at least one processor configured to execute: a receiving unit that receives a request for registration of a registration candidate document from a new registrant; a word extracting unit that extracts a word from the registration candidate document; a registrant information acquiring unit that acquires information on the new registrant; and an associating unit that associates the word extracted from the registration candidate document with information indicating a group to which the new registrant belongs; at least one memory comprising: a first storage unit that stores history information, the history information containing a registered word that is extracted from a document previously requested to be registered and an identifier of a previous registrant who registered the registered word, in association with each other; and a second storage unit that stores the identifier of the previous registrant and information indicating a group to which the previous registrant has belonged, in association with each other; wherein the at least one processor is further configured to execute: an extracting unit that reads the history information from the first storage unit, extracts an identifier of a previous registrant who previously registered a registered word identical to the word extracted from the registration candidate document, on the basis of the history information, and extracts a group to which the previous registrant has belonged, on the basis of the identifier of the previous registrant and association stored in the second storage unit; a registration permission determining unit that determines whether to allow registration of the registration candidate document, on the basis of whether the group to which the new registrant belongs is included in the group to which the previous registrant has belonged; and a document registering unit that registers the registration candidate document, if the registration permission determining unit determines to allow the registration.