The aim of a testing group is to:
Test content variability
When creating an e-mail campaign, you may personalize its content. The customization can be simple (inserting the name or the first name of a person) or more complex (showing some blocks only for profiles who fit specific criteria). In order to test the display of each version of your e-mail, your testing group must contain a sample of all the different possibilities.
Simplify the validation process of an e-mail
By including your colleagues/managers in the testing group, sending test e-mails will be easier and quicker. You will also benefit from real-time tracking by learning directly if the test e-mail was opened, clicked on,...