##構成
Memberテーブル
Registrantテーブル

この二つを繋げるための、
中間テーブル Recruitmentテーブル

Recruitmentテーブルのカラム
recruiter_id(モデル名と違う)
registrant_id(モデル名と同じ)

app/model/member.rb

class Member < ActiveRecord::Base
  has_many :recruitments, class_name: "Recruitment", foreign_key: "recruiter_id"
  has_many :registrants, through: :recruitments
end

app/model/registrant.rb

class Registrant < ActiveRecord::Base
  has_many :recruitments
  has_many :recruiters, through: :recruitments
end

app/model/recruitment.rb

class Recruitment < ActiveRecord::Base
  belongs_to :recruiter, class_name: "Member", foreign_key: "recruiter_id"
  belongs_to :registrant
end

migration

  t.references :recruiter, null: false
  t.references :registrant, null: false

add_index :recruitments, [ :recruiter_id, :registrant_id ], unique: true
add_foreign_key :recruitments, :members, column: "recruiter_id"
add_foreign_key :recruitments, :registrants