It's easy to add more testers to your test. This help article explains how.
Adding testers requires credits: To add more testers, you will need credits available in your account balance. If you need to purchase more credits, you can visit the Upgrade Your Plan / Billing Section to buy a credit package or upgrade your subscription.
Tests with Manual Review of Applicants
For tests with manual review of applicants in your screening survey, you can add more testers just be Accepting more testers from your Pending list of testers. You will need to have credits available within your account.
Tests In progress (with Automatic Acceptance)
For single session tests with Automatic Acceptance set in the screening survey, you can add more testers during a test by clicking the "Add more" link to the right of the test progress indicator on the test home page.
Clicking this link will show you the additional cost in credits for adding X more testers:
Tests In Complete / Review Stage
After ending a test, you can add more testers using the feature available on the test home page. The right section "Continue Testing" allows you to click a button to add more testers.
You can't add testers to fully closed tests: Once a test has been fully closed, you can no longer add more testers. You would need to duplicate the test and run a new test.