group_removed_users table to forbid users joining once removed

db/migrate/20190716173227_create_group_removed_accounts.rb

@@ -0,0 +1,12 @@
+class CreateGroupRemovedAccounts < ActiveRecord::Migration[5.2]
+  def change
+    create_table :group_removed_accounts do |t|
+      t.belongs_to :group, foreign_key: { on_delete: :cascade }, null: false
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }, null: false
+      t.timestamps
+    end
+
+    add_index :group_removed_accounts, [:account_id, :group_id], unique: true
+    add_index :group_removed_accounts, [:group_id, :account_id]
+  end
+end