Starting in v3.0 you can create custom watchlists to help group your stocks.
To make a new watchlist, open the app and click on the '+' button in the bottom left corner.
A new untitled watchlist will appear in the sidebar.
You can edit its name and choose an icon from the list.
At any point you can edit a watchlist name or icon by selecting it in the sidebar and pressing on the gear button in the bottom left corner (or right clicking on the watchlist itself).