The Portal is generated by the Portal plugin, so with the right company selected visit Settings > Company > Plugins and click to Manage Portal. Modify the HTML as desired and save. Change to the other companies and do the same. Make sure the hostnames are set correctly under Settings > System > Companies. If you want to modify more than just the content, then you will need to clone the client template directory /app/views/client/bootstrap/ to rename it for each company, then under each company select it under Settings > Company > Look and Feel > Template, and modify the files as desired. Rather than copying the entire directory, you can just copy the files you intend to change as it will fall back to the bootstrap template for any files that are missing, which will make it easier to upgrade in the future.