A problem has emerged in another thread that points to them operating a one bonus per group policy. This means that the mailer could be misleading as it offers the "New Player" sign-up bonus. If the recipient has already played at a different Grand Prive skin in the past, they will not be eligible for this offer, and if they use it they will have their "New" account locked.
If you have NEVER played there before, at any of the skins, then take the free money if it involves no deposit, after all, it's a risk free bet. IF you go on to make deposits, make sure you are eligible for any bonuses, and that you have the means to WITHDRAW, which is a lot harder than making deposits. This includes being able to pass verification by sending documents, and getting them approved.