def add_member(user, group)
      relation = group.group_member_relations.find_or_initialize_by(user: user, accepted_at: nil)
      relation.accepted_at = Time.now
      relation.accepter = self
      relation.save
    end
def add_member(user, group)
      relation = group.group_member_relations.find_or_initialize_by(user: user, accepted_at: nil)
      relation.accepted_at = Time.now
      relation.accepter = self
      relation.save
    end
TODO: had better check!