ruby on rails - How to get params in controller from collection_select -

i try handle data collection_select in view :

    <%= form_for(:group, :url => {:action => 'update', :group_id => @group.id}) |f| %>                    <th>new member</th>             <td><%= f.collection_select(:members, @users, :id, :first_name) %></td>             <%= submit_tag("update group") %>     <% end %> 

but i'm not able in controller :

member_id = params[:member] puts member_id.inspect 

inspect result in console :

nil

but when error, can see member_id exists :

undefined method `each' "1":string      "utf8"=>"✓",      "authenticity_token"=>"gxcd==",      "group"=>{"name"=>"g7",      "members"=>"1"},      "commit"=>"update group",      "group_id"=>"4" 

thanks !

if take @ error posted, can see params following hash;

{     "utf8"=>"✓",     "authenticity_token"=>"gxcd==",     "group"=> {         "name"=>"g7",         "members"=>"1"     },     "commit"=>"update group",     "group_id"=>"4" } 

params[:group] nested hash, access members key params[:group][:members]