To add additional users as test users on your test Apple Messages for business account, perform the following steps:

1. Navigate to the Apple Business Register page.
2. Log in to your account.
3. Select your Messages for Business Account.
4. Locate the Testers Apple Accounts section and select Edit.
5. Click Add Apple Accounts and add one or more Apple Account IDs for testers to use. Click the image to enlarge.

As a tester, if you are not able to send messages successfully while testing and if this persists for more than 24 hours, perform the following steps:

1. On the Apple device that you are using to send messages, navigate to Settings > Apps > Messages.
2. In the Messages screen, disable the iMessage toggle.
3. Restart the device.
4. Browse back to Settings > Apps > Messages and enable the iMessage toggle.
   Note: If the issue persists, contact Apple support. Ensure to provide the business messaging ID and the tester Apple account details.