Is it ok to put that title on LinkedIn or does that require the enterprise to be officially declared ?
You can put whatever you like on LinkedIn, just like you can give yourself whatever title you want in an organisation you control, whether said organisation is real or fictitious. But what you should be asking is what is considered appropriate or professional.
Since the owner of a business has full control, he can give himself whatever title he fancies and there are indeed plenty of people who introduces themselves as CEO or President of their one-man operation. Because it's become so common I can't outright tell you that it's a faux pas or give a blanket recommendation against it. Certainly in start-up cultures people might expect that you call yourself that even if your company never really came off the ground.
However, many hiring managers will still roll their eyes when they see CEO and "one-man business" in the same application because it's seen as self-aggrandizement. I would recommend that you opt for the simpler, less pretentious and slightly more accurate "Founder" instead.
Even in your case where you're dealing with 8 people coming together to launch the company I would still be hesitant to adopt CxO labels, just because they're so often used by people in poorly run start-ups to inflate the importance of their work. It can be difficult to justify using executive titles in a small company, though as mentioned it's becoming more common to do so. As mentioned above doing so probably won't really hurt your profile but I think a little modesty in calling yourself simply a (co-)founder and explaining your role in bullet points and accomplishments rather than via a title would be the way to go.
The fact that the company isn't officially incorporated yet is ultimately not relevant, provided that you don't misrepresent the effort involved. If you are working on this business plan full-time the lack of incorporation isn't an issue. But if this is all still in the planning stages and you only spend a few hours a week on this you most likely don't want to present this on the same level as your actual nine-to-five job.
The main thing to keep in mind is that you want to highlight this experience on your resume without giving the appearance that you're embellishing. Resume writing is a skill and takes a careful balance between overselling and underselling but people who are ahead of the curve when it comes to experience, like you are in this instance, should typically opt for the latter and let their experience speak for itself.
Do note that all this only applies if you are or will be job searching after graduation. If you want to use LinkedIn more to promote your new organisation or your place in it then I can see an argument being made to opt for CxO labels from the start.