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Well, it depends. If you only enable access to the demo without storing any personal information, there is no prblem whatsoever.

If, in order to access the demo, you need to give your e-mail address, and you are harvesting e-mail addresses in this way, you need to inform the users you are doing so, and provide a separate unchecked box "Subscribe to the Newsletter". In this way you are honest with the users, with how you are using their data, and you stick to the letter and the spirit of the law.