Here are some of a tester's activities on an XP project (we'll provide more details in Part II: Test Drive through an XP Project):
Help negotiate quality with the customer.
Help clarify stories.
Help with estimates during planning.
Advocate the customer's rights.
Guard the programmers' rights.
Work with the customer to write effective and thorough acceptance tests.
Make sure acceptance tests (functional, load, stress, performance, compatibility, installation, security, anything not covered by unit and integration tests) verify the quality specified by the customer.
Help the team automate maintainable acceptance tests, using lightweight tools and test designs.
Make sure test results are reported in a timely manner, forming a continuous feedback loop for the team.
Make sure acceptance testing keeps pace with development.
Help the programmers design more testable code.
Keep the team honest; if they sacrifice quality to speed ahead, they must reveal this to the customer.