Remove addUnregisteredMember unite with addMember

Because of it in a controller-layer logic, not a model.

Locate the addUnregisteredMember and addMember implementations in the controller-layer logic, then compare how they interact with the model. Unify the two operations as described and remove the separate unregistered-member path; verify that the resulting controller behavior still handles both member cases.

索引モデルが issue の本文から書いたものです。