Can Task Manager show the total memory usage for a group of processes?

Use PowerShell:

• (Get-Process chrome | Measure-Object WorkingSet -sum).sum
• (Get-Process tweetd* | Measure-Object WorkingSet -sum).sum

Process Hacker shows memory usage grouped per process on its Tray Icon and added the Option to the Main Window as per user request in January 2016. Here's an example screenshot:

The Main Window Option is configured in:

Options->Advanced->Include CPU (and other) usage of children in collapsed processes

With this option enabled, when you collapse a process tree, you see metrics for its entire tree.

Microsoft added this capability to Task Manager as of the Insider build 16226. The feature will go out to everyone with the next major update of Windows 10, scheduled for September/October 2017.

Update: Doesn't work since Chromium 51.

Enter about:memory in Google Chrome and you'll be redirected to chrome://memory-redirect/

That page has totals for all popular running browsers:

Google Chrome's about: Pages