How do you ensure that people who are joining the group are really interested in investing or have enough capital to invest?
They will have to prove it at one point or another, e.g. sign a message from a wallet. Even if they don't have the money right now, they might be able to crowdfund it or raise it, so it doesn't need to be proven right away.
My personal concern is not the funds, it's easy to provide evidence for having them.
What's worries me a bit is how you ensure that any idea that will be discussed and reveal in the group will not be used from any other participant?
I mean without the knowledge of the author of this idea. That's not so easy.
I agree. Even though idea by itself is not everything - but then somebody else with proper contacts or moolah can implement it before you can. I'd see if I join this.