what are the private investor platform you mentioned ? and what are the requirements to join in?

Hiive and Forge Global are the ones I know of. You must be an "accredited investor" which means nothing at all except that you have a million dollars or make $200k/yr.

Like you can buy shares of Anthropic as long as you prove you make over 200K? That easy? Shouldn't they approve of the purchase? Sorry, noob in this space!

They have to approve and it's not as simple - it's just that if you make $200k a year or have $1m in the bank, the government assumes you're a knowledgeable investor and allows you to bypass certain protections.

If you are NOT knowledgeable and simply have money ... well it'll soon be parted.

The secondary platform verifies you and then you indicate interest. If there’s a seller you may get to buy. Company may ROFR. Priority goes to bigger buyers.

EquityBee got me investors to exercise my Brex options, in exchange for giving up some small beta

I don't remember which one I was looking at - they all have different requirements.