To group markers together under a file, click the file icon in the top right corner of the overlays panel.
(If you do not see it, click on the serrated edge on the right to bring up the overlay panel.)
Clicking on the pencil icon beside the file will bring up a pop up to change the File Name.
Normally the folders are self organized as "Group a", "Group b", etc. but you may edit the naming by clicking on the pencil icon.
Beside each element in the Layers panel, there is 3 dots. By clicking on these, you will bring up choices to either create a copy of the element or move the element to a group folder.
By clicking on "Move to Folder" a popup will appear with the different group folders available. Choose the folder you like and then click on it. The item will now appear within the folder.
It is possible to re-arrange the folders by dragging and dropping: