Underpaid at a startup, but like everything else about the job. Advice?

Underpaid according to what? Do you have other offers that pay better? Do you know that other people doing the same work at the startup get paid significantly more? Presumably you accepted an offer but now think you should get paid more. Based on what? You don't have to explain to me, but you will have to explain to your employer.

If you think your employer pays you less than you deserve, or could get elsewhere, you ask for more money. You should make a case for yourself, and prepare for getting no raise, or less than you want. What other advice would you need?

I will caution that asking for a raise based on what other people in the company make usually doesn't go over well. Everyone gets paid what they negotiate and ask for, not according to some industry-standard pay scale. Typically you leverage another offer (or the strong possibility of getting one).