Intra user messaging

_form.html.haml

= simple_form_for(@memo) do |f|
  - if @memo.errors.any?
    #error_explanation
      %h2
        = pluralize(@memo.errors.count, "error")
        prohibited this memo from being saved:

      %ul
        - @memo.errors.full_messages.each do |msg|
          %li= msg

  .inputs
    = f.input :content

  .actions
    = f.button :submit

index.html.haml

%h1 Listing memos
    
- @memos.each do |memo|
  %ul
    %ul
      %li= memo.sender.login
      %li= memo.content
      - if memo == @memos.last
        = render 'form'

%br

memos_controller.rb

class MemosController < ApplicationController
  # GET /memos
  # GET /memos.xml
  def index
    # change the index to only show chronological messages between the current user and the other user
    @memos = Memo.where(
      "(sender_id = ? or receiver_id = ?) and (sender_id = ? or receiver_id = ?)",
      current_user, current_user,
      params[:other_user],
      params[:other_user]
    )

    @memo = Memo.new
    @other_user = User.find(params[:other_user])

    respond_to do |format|
      format.html # index.html.erb
      format.xml  { render :xml => @memos }
    end
  end

  # POST /memos
  # POST /memos.xml
  def create
    @memo = Memo.new(params[:memo])
    @memo.sender = current_user

    respond_to do |format|
      if @memo.save
        format.html { redirect_to( :action => "index", :other_user => params[:other_user]) }
        format.xml  { render :xml => @memo, :status => :created, :location => @memo }
      else
        format.html { redirect_to( :action => "index", :other_user => params[:other_user]) }
        format.xml  { render :xml => @memo.errors, :status => :unprocessable_entity }
      end
    end
  end
  
end

I feel its because you are not passing the params[:other_users] from your form to create method.
I am not sure about haml syntax but if you want to pass params[:other_user] from your form to create method, you can use form_tag.
I have used it many times.
Alternatively, you can create a temp table and store value of params[:other_user] in that table when you are in action 'new' and retrieve it when you are in action 'create'.

Hope this helps!
cheers.