Yes, you can group your websites. It is very useful if you have hundreds of even more of websites and want to separate them into groups.
You can do it by clicking on the "Groups" tab when creating or editing a monitor.
You can filter your monitors by a group in 2 ways once it is assigned to a group.
The first way is in the dashboard. You have to click on the "Filter" button and then "Select groups".
The second way is when viewing all of your created monitoring checks. You will find a button "Filter by group" on the left-hand-side menu.