Here are some test-specific activities for the startup phase of an Agile project.
Think about managing the test efforts and results in the Agile process. Will you write many test cases (in detail) - which might demand a lot of effort to maintain as requirements are changed during the project. Personally I would recommend something like session based test management which is very lightweight in documentation but perfect for a hands-on testing approach in Agile. With a proper dashboard it's quite easy to follow up key metrics as test manager.
Prepare your test automation framework
As you have already decided on the tool, a proof of concept is no longer necessary. You should, however, consider setting up a framework, that will allow for swift test creation and decent reporting (which isn't always easy). Try to decide on key factors such as the format of your test data (in case you would need a database, now's the time to get it arranged).
Make agreements about test automation
I don't know FitNesse, but here we use Selenium. So an important agreement with the developers is that they provide ID attributes to elements where possible, to make the automation process much quicker.
Make agreements about testing in the Agile process
Sit together with your team and decide on how you will deal with testing in sprints. For example:
- Definition of done
- do automated tests have to be written to complete a story?
- what amount/level of open bugs is acceptable to complete a story?
- acceptance criteria are a must: why not have developers check them together with the analyst before handing a story over to the testers?
- What are the bug fixing priorities for developers (fixed time slot every day, zero-bug policy, ...).
- Continuous integration: if possible, you as tester should be able to decide when to pull checked-in functionalities into your testing environment.
You might want to provide some training or exercises for the testers, e.g. to get familiar with FitNesse, SQL or exploratory testing.