Account groups are for helping Rate→Agent rate to manage the rate. We put accounts with the same policy into a group, and bill them altogether. Click Add and the page pops up.
Tick the accounts you would like to add/remove, and drag them leftwords or rightwords.
As shown in the picture, the red process is add to the group, and blue one is to remove the accounts from the group.
And click save to save the result.
Save and a new account group has been set up.