Earlier investors usually have the option of reinvesting (at the new valuation) in order to maintain their proportion of the company. Typically, they also get the same terms (which tend to propagate to all the investors in the round.) This is why taking shitty terms early on can damage you, even if you think the 5x liquidation preference you took on that $1 million A round "shouldn't" matter. On its own, it's only $5 million, but you're likely to face MLP (and participating preferred, which is also horrible) in all future rounds.

Founders and employees do not get to reinvest. Typically, when a VC-funded company is allowing employees to buy more equity is the last time to take that deal (it means the company is cash-poor and in bad shape). General rule: unless you're a founder, avoid taking the other side of any deal with VCs in it.