Imo, it cannot be the career suicide. Let's imagine the worst case for you, where your startup fails and you don't get any good experience from it. Even in this occasion you cannot worsen your current kase(knowledge, abilities, skills and experience), so if you don't gain anything from working on startup(and i'm sure, you will), you probably won't cause any damage to your current CV at all.
From another point of view, let's think about what you may lose and the probability that you lose it if because of missing the opportunity to work on the startup. The multiplication of those two gives you a mathematical expectation on what you get from actually joining it.
I'd personally recommend you to draw a Descartes Square to think of these decision since i don't know any detail about what work you are doing and what kind of job you are supposed to do in the startup as well as i have no idea about the startup itself.
Another recommendation would be to find a highly qualified person in your branch of work, who is neither connected with your job nor the startup, and ask them about their opinion on your particular case. It would be a good idea to ask your colleagues and friends about someone in the industry, you are working at reasoning it with "I want to ask how everything works in the industry besides my company" or you can actually state the exact reason in detail.
To sum up, do what you feel like doing, because you cannot significantly succeed doing something you do not like.