1. A job log processing apparatus comprising: a processor; a memory; and a collection unit that, executed on the processor, collects job logs, inclusive of information of identification of users having requested jobs, time of execution of each job, and a number of pages printed, from execution devices by which the jobs have been executed; a job log storage unit that, executed on the memory, stores the collected job logs; a section information storage unit that, executed on the memory, stores information of a group of users to which the users belong respectively; an update unit that, executed on the processor, updates the information of the group of users stored in the section information storage unit based on change information indicating a change of the group of users to which the user belongs to create user change historical data, the user change historical data indicating a date of the change of the group of users and the change of the group of the users; a record creation unit that, executed on the processor, extracts a user identifier from each of the job logs stored in the job log storage unit, acquires the information of the group of users from the user change historical data based on the extracted user identifier; a change historical data creation unit that, executed on the memory, creates a user change historical data which includes (i) the user identifier, (ii) the time of execution of the job, and (iii) the information of the group of users indicated by the job log as the group to which the users belong at the time of execution of the job, wherein (i) the user identifier and (ii) the time of execution of the lob are extracted from the job log, and wherein (iii) the information of the group of users indicating the group to which the users belong at the time of execution of the job is acquired based on the information of the group of users and the job log, an association unit that, executed on the processor, associates the number of pages printed of the job log with the group of users based on the group of users acquired from the user change historical data with respect to each job log; and a control unit that, executed on the processor, aggregates the number of pages printed for each group of users based on the user change historical data and each job log, wherein, before a user changes a group of users from a previous group to a new group, the control unit aggregates the number of the pages printed as a usage of the user belonging to the previous group, and wherein, after the user changes the group of users from the previous group to the new group, the control unit aggregates the number of the pages printed as a usage of the user belonging to the new group.