Account groups allow you to organize accounts - you can create groups based on the frequency of use ("Everyday accounts", "Savings - Do not touch!"), institutions ("Chase accounts", "Lloyds accounts") or any other criteria you might come up with.
To create an account group, you should click on the account group icon at the bottom of the accounts list (the middle one).
You'll be asked to indicate the group name, group color and the group currency. The group currency will be used to display the group total - the sum of all accounts stored within (only those selected to be included in Net Worth calculation). You'll also be able to select which accounts to put in a group by selecting them from the list.