None. Early investors often have a ROFR (Right of first refusal) in their share agreement, in which case they will have the right to buy shares proportional to the amount of dilution they would otherwise suffer (so they can maintain their % stake in future funding rounds) but I've never heard of an employee having that right.

Also note early investors often have a liquidation preference meaning they get their money out before subsequent investors but this also isn't something which is usually extended to employees.

You will get diluted beyond all belief. Not just by investors but by future grants to employees. The best that you can hope for is that that dilution corresponds to growth in the equity of the company such that you own a smaller share of a larger pie and in overall dollar value your stake becomes more valuable or at least maintains its value. It's worth mentioning that in the vast majority of startups this does not end up being the case and equity ends up not being that valuable. There is a very small probability of a lifechanging exit though which is why people go for it.