1. A system for assisting a collaborative activity, comprising: a plurality of work interfaces configured to provide work environments for shared information that is shared in the collaborative activity; a role management unit configured to manage user information obtained by defining for each user a role in the collaborative activity and an authority regarding the shared information; a workflow management unit configured to manage activity statuses by using a workflow defined by activity contents and a sequence thereof; a collaborative activity registration unit configured to register collaborative activity information regarding the collaborative activity, to which a collaborative activity identification value is assigned; and an activity context management unit situated between the plurality of work interfaces and the collaborative activity registration unit to receive entered information from one of the work interfaces, to associate the shared information contained in the entered information with the user information provided by the role management unit and an activity content provided by the workflow management unit to create the collaborative activity information, and to cause the collaborative activity registration unit to register the collaborative activity information, wherein the collaborative activity information includes an activity content identification value indicative of an activity content performed on the shared information, and includes a user identification value and a role identification value of a user who has performed the activity content, and wherein in a case of performing a new activity content on the shared information, the shared information that is most recent and a first activity content identification value contained in the registered collaborative activity information are obtained based on the collaborative activity identification value, followed by obtaining, from the workflow, a second activity content identification value indicative of an activity content that is to be performed next following the activity content indicated by the first activity content identification value with respect to the most recent shared information.