First what you would want to do is go to 'My Collections' from the user dropdown menu and click on the Create + button to create a Collection.
You'll want to use select these settings:
Privacy Public
Collection Type Model
Name and Description are up to you!
From there, you'll want to save all of the models you want to group together into this Collection. You do this by clicking the Save/Bookmark icon on the model at the top or in the model details drop-down.
Once you have the model saved to the Collection, click Edit Model and scroll down to the Showcase Collection option.
Select 'Click here to select a collection', and it will allow you to select the Collection.
You do have to do this individually with each model.
When finished, it will display on the model card for each model that you've opted in to Showcase Collection.