search form: POST vs GET

You seem to be trying to partially use restful design. I think that
the "rails way" would be to use a routing like this:

map.resources :groups do | group |
   group.resources :members
end

And then have a restful controller design for the members controller.

Instead of a special add_members action you would use the normal rest actions

class MembersController < ApplicationController

  before_filter :load_group

  def load_group
    @group = Group.find(params[:group_id])
  end

  # GET /groups/1/members/new
  # GET /groups/1/members/new.xml
  def new
    # get a list of members to present to the user to add to the group.
    # the actual set of members which can be added is determined by
    # business logic in the group model.
    # The form rendered by the new action should set params[:id] to the
    # id of the member selected. It also needs to set params[:group_id] to
    # the id of the group. The best way to do this is via a hidden
field on the form

    @members = @group.potential_new_members

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

  # POST /groups/1/members
  # POST /groups/1/members.xml
  def create

    respond_to do |format|
      # The group model should have a method to add a member, possibly with
      # some validition. It should return false/nil if the add operation fails
      if @group.add_member(Member.find(params[:id]))
        flash[:notice] = 'Member was successfully created.'
        format.html { redirect_to(group_member_path(@group, @member)) }
        format.xml { render :xml => @member, :status => :created,
:location => @member }
      else
        format.html { render :action => "new" }
        format.xml { render :xml => @member.errors, :status =>
:unprocessable_entity }
      end
    end
  end
end

The url to prompt for a new member to be added to group 1 would be

   /groups/1/members/new

If you really wanted to allow /groups/1/addmembers you could add an
additional route.