You can add a sub-account by navigating to Settings → Accounts, then click New Account in the upper right.
There are a few key pieces of information to get your new sub-account created:
- Name the account
- Select the Business Industry
- Choose how you want to bill the charges for the account
In the billing options, you can choose to put the new sub-account on the same available balance and credit card as your Agency. Or, you can invite your client to enter the credit card for the account, and they will be responsible for their numbers and minutes. If the person adding the credit card is not already a user in the account, they will be automatically added to the Billing role.
If you ever need to change the billing option for a sub-account, go to Settings → Accounts and click the text in the "Balance From" column for that sub-account. It will bring up a pop-up box where you can remove an account from agency-shared billing or put it on shared billing.
Once the account has been added, you will see it listed on the Accounts page, available from the Settings menu. You can also switch between accounts from this section by clicking on the eyeball next to each account's name in the first column.