When you need to give your end-users access to a specific sub-account they'd need to use multiuser credentials in order to access this sub-account. The multi-user feature must be enabled on the parent account.
Below you'll find the steps to set it up:
1. Log in to your "parent" Flowroute account and opt in for the "Multi-user" feature. Once the Flowroute team has enabled it for you, the "Features" page on the Manage portal will look similar to the screenshot below:
2. Navigate to the "User Management" page under "Advanced" preferences. Here you will need to fill out the end-user information and grant them privileges. What permissions you grant is completely your decision. For example, if you only want them to be able to access billing on one account, please set "Role" to the "General User" and select the appropriate sub-account for this role.
More information about sub-account and multi-user features can be found here.