A method for providing a closed messenger service in a server, the closed messenger service comprising workspaces including a target workspace, wherein first-authority-level users are authorized to access the closed messenger service and are able to request access to the closed messenger service for second-authority-level users who are not otherwise authorized to access the closed messenger service, the method comprising: receiving, from a first terminal, (i) an invitation from a first-authority-level user inviting a second-authority-level user to join the target workspace of the closed messenger service, (ii) a first identity identifying the first-authority-level user, (iii) a second identity identifying the second-authority-level user, (iv) information identifying the target workspace to which the second-authority-level user is invited by the first-authority-level user, and (v) information about a usage period of the target workspace; determining, based on the first identity, that authorization is required for the invitation to grant the second-authority-level user access to the target workspace, and based thereon transmitting an authorization request to an approver; receiving, from a second terminal, an authorization confirmation granted by the authorizer based on the authorization request; based on the authorization confirmation, the second identity, and the information identifying the target workspace, performing a workspace member registration procedure that registers the second identity as a member of the target workspace; deleting the second-authority-level user from members of the target workspace responsive to the usage period of the target workspace expiring; and deleting an account of the second-authority-level user from the closed messenger service responsive to the second-authority-level user being determined not to be registered as a member of another workspace of the closed messenger service on the basis of the second identity, and transmitting an account deletion notification using the second identity. receiving, from a first terminal, (i) an invitation from a first-authority-level user inviting a second-authority-level user to join the target workspace of the closed messenger service, (ii) a first identity identifying the first-authority-level user, (iii) a second identity identifying the second-authority-level user, (iv) information identifying the target workspace to which the second-authority-level user is invited by the first-authority-level user, and (v) information about a usage period of the target workspace; determining, based on the first identity, that authorization is required for the invitation to grant the second-authority-level user access to the target workspace, and based thereon transmitting an authorization request to an approver; receiving, from a second terminal, an authorization confirmation granted by the authorizer based on the authorization request; based on the authorization confirmation, the second identity, and the information identifying the target workspace, performing a workspace member registration procedure that registers the second identity as a member of the target workspace; deleting the second-authority-level user from members of the target workspace responsive to the usage period of the target workspace expiring; and deleting an account of the second-authority-level user from the closed messenger service responsive to the second-authority-level user being determined not to be registered as a member of another workspace of the closed messenger service on the basis of the second identity, and transmitting an account deletion notification using the second identity.