show the user the group they originally selected if possible when editing group

else fallback to any (oldest) org in the group

app/models/organisation.rb

@@ -258,6 +258,21 @@ class Organisation < ApplicationRecord
     Organisation.visible.where(group:).order(:created_at).first
   end
 
+  # Shown on the edit organisation page to indicate which group member the user initially entered, if possible
+  def selected_group_member
+    return nil if oldest_group_member.nil?
+
+    # find organisation by group_member_id if it exists
+    selected_group = Organisation.visible.find_by(id: group_member_id)
+
+    # ensure that its still in the same group
+    if selected_group.present? && selected_group.group == group
+      selected_group
+    else
+      oldest_group_member
+    end
+  end
+
 private
 
   def validate_profit_status

app/views/organisations/edit.html.erb

@@ -42,7 +42,7 @@
                 :id,
                 :name,
                 label: { text: "Search for an organisation that is part of the same group as this organisation", size: "m" },
-                options: { disabled: [""], selected: @organisation.oldest_group_member&.id || "" },
+                options: { disabled: [""], selected: @organisation.selected_group_member&.id || "" },
                 "data-controller": %w[accessible-autocomplete conditional-filter] %>
           <% end %>
           <%= f.govuk_radio_button :group_member, false, label: { text: "No" } %>