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.