Background: I'm a founder who's helped multiple friends negotiate offers with early stage companies.
When you join this early, the main compensation is equity.
To give you a benchmark, most investors pressure founders to never grant more than ~1% of the company to a single employee. In this current environment though, you could probably push for ~1.5%, especially if you take the title of "founding engineer".
When it comes to salary though, you can do three things:
1. Clarify the minimum you'd "happily" take to work with them. This mainly depends on your alternatives and how much you care about this company
2. Understand how much budget they have by asking how many people they want to hire to achieve what milestone when. Note: often people don't have a plan, so you have to resort to no. 1
3. Negotiate a written agreement to raise your salary by X% when the next fundraising goes through
Finally, look for ways to enlarge the pie, such as negotiating getting a 1on1 with the founders every 2 weeks to learn about whichever topic interests you.
Did I forget anything? Happy to share more.