Thanks for reaching out to the Community, @Rudolf.
To start off, make sure that they are already added or set up as clients in your QuickBooks Online Accountant (QBOA). Here's how:
1. Go to Clients menu at the left under Your Practice.
2. Click Add client and pick whether the client is a Business or Individual.
3. Enter the necessary information, then under QuickBooks subscription, select whether you or the client will pay for the subscription.
4. Choose Compare, then pick a product.
5. An email will be sent to your client so they can set up a user account.
6. Hit Save.
Once done, here's how to add them to your Wholesale Billing account:
1. Go to the Gear icon at the upper right, then pick Your Account under Your Company column.
2. Click Add existing clients, then check the boxes for the clients to add.
3. Select Add clients.
4. Read the notification, and Continue.
For additional information, check out this article: Add or remove clients from Wholesale Billing.
That should do it. Please let me know how it goes or if you need anything else. I'll be here to help. Have a great rest of the week.