You become a WTB (want-to-be) team member once all your test tasks are finished, and the speed and quality of that finish is liked :} I think there are two test tasks total, though there may be more sometimes. Once you are a member you continue with assignments, but you also have all information about development - what goes on, what the plans are, etc. When you are a prospect, you're in an application stage where your work is judged for the final decision.
WTB members become full developers once they have proven their dedication and skills - likely to take a few years :}
