Client, server, and information sharing system

A server having a communication function with each of a plurality of clients serving as mobile stations, the server comprising:

• a database which associates and stores a group action ID of a group action and a user ID of each of members serving as users registered to participate in the group action, and associates and stores a condition for the members to start the group action as a first condition and the user ID;
• and
• a server control device configured to determine whether or not the first condition is satisfied for each of the members based on the first condition stored in the database, and,
• if the first condition is determined to be satisfied for one of the members, transmits, to the client used by the member, an order for causing an input device to function as an input interface for sending predetermined first information associated with a predetermined input mode by the predetermined input mode having a smaller number of times of input or a smaller information quantity to be input with respect to the input device, which is provided at the client used by the member, than a case in which the server control device determines that the first condition is not satisfied for the member.