First, Groups and Scenes was actually "un-deprecated," so you can still create new instances even if you've never used the app before (and even deprecation didn't meant that it would stop working or that you couldn't if you already had it). It's unlikely to receive new features or much/any further development, with Room Lighting being the more-featured suggestion as a replacement, so that much is at least true.
To answer your question, what you need to do in Room Lighting is create a name for an "Activator Device" at the bottom of the main page:
This is like the Group or Scene activator that gets created automatically (with the name of the group or scene). Commanding it will command all of the devices in the Room Lighting "group," and similar to Groups and Scenes, there are a few options that control exactly how it works. See here for more: How to use Room Lights for Groups
