The account groups that you have created appear in the account selectors on both Dashboard, Account overview, Transactions and Payments page under the tab "Groups".
To use one of the account groups, simply click the name of the group. When you click the group, the page or widget will be updated to show data for the accounts in the account group. If you swith to the "Accounts" tab, you will see that the account in your group is selected here.
Please note, that the "All accounts" group is a generic group that cannot be deleted.