There are 3 ways of including an account into a group:
While reordering the accounts, put them under an account group name.
Modify the account, switch to Extended tab and use the "Put in Group" field.
Modify the account group and use the list to put new accounts into the group.
Remember that accounts do not have to belong to a group - it's optional. If you have account groups, accounts that are not part of any group will always be listed at the top of the list.