1. A group forming method performed by a computer by executing a program to perform a process, comprising: extracting a plurality of keywords from public information of a first group of contributors including a contributor for which an attribute value regarding a first attribute is calculated as corresponding to a predetermined attribute value, the first attribute being an attribute with respect to each contributor of the public information and an attribute value of the first attribute varying with respect to passage of time; forming a first contributor group set by forming a contributor group of the public information containing a corresponding keyword with respect to each keyword contained in the plurality of keywords; forming a second contributor group set with respect to new public information by forming a contributor group of the new public information containing a corresponding keyword based on each keyword contained in the plurality of keywords; identifying a contributor group of which a distribution of an attribute value of an attribute different from the first attribute indicates a predetermined similarity relationship from among contributor groups contained in the second contributor group set that are formed by using the same keyword as a keyword of each contributor group contained in the first contributor group set; and forming a new contributor group corresponding to the first contributor group by using a contributor whose attribute vale regarding estimated the first attribute is calculated as corresponding to the predetermined attribute value from among contributors contained in the identified contributor group.