And add to group

app/controllers/groups_controller.rb

 class GroupsController < ApplicationController
   include GroupsHelper
   before_action :set_group, only: [:show, :edit, :update, :destroy]
-  before_action :require_admin!, only: [:index]
+  before_action :require_admin!, only: [:index, :process_mass_add_members, :mass_add_members]
   before_action :require_membership!, only: [:show]
   before_action :require_leader!, only: [:edit, :update, :destroy]
 
     end
   end
 
+  def mass_add_members
+    @group = Group.find(params[:group_id])
+  end
+
+  def process_mass_add_members
+    @group = Group.find(params[:group_id])
+    require 'csv'
+    uploaded_io = params[:spreadsheet]
+    result = Person.from_csv(uploaded_io.read)
+
+    result.each do |p|
+      m = Member.find_by(person: p, group: @group)
+      if not m
+        m = Member.new(person: p, group: @group)
+        m.save!
+      end
+    end
+    flash_message(:success, "#{result.count} people added to group")
+    redirect_to group_members_path(@group)
+  end
+
   private
     # Use callbacks to share common setup or constraints between actions.
     def set_group

app/views/groups/mass_add_members.html.haml

+.container
+  .row
+    .col-md-12
+      %h1
+        Mass-adding members to
+        = @group.name
+      = form_tag("/groups/#{@group.id}/mass_add", method: 'post', multipart: true) do
+        = file_field_tag 'spreadsheet'
+        = submit_tag

config/routes.rb

     get 'invite', to: 'members#invite'
     post 'invite', to: 'members#process_invite'
 
+    get 'mass_add', to: 'groups#mass_add_members'
+    post 'mass_add', to: 'groups#process_mass_add_members'
+
     resources :members do
       post 'promote', to: 'members#promote', on: :member
       post 'demote', to: 'members#demote', on: :member