canvas-lms/app/messages/added_to_conversation.summary.erb

<% define_content :link do %>
  http://<%= HostUrl.context_host(asset.context) %>/conversations/<%= asset.conversation_id %>
<% end %>
<% define_content :user_name do %>
  <%= asset.author.short_name_with_shared_contexts(user) rescue t(:unknown_user, "Unknown User") %>
<% end %>
<% define_content :subject do %>
  <%= t :subject, "%{user_name} just added you to a conversation in Canvas.", :user_name => content(:user_name) %>
<% end %>

<%= t :body, "%{user_name} just added you to a conversation in Canvas.", :user_name => content(:user_name) %>