We’re doing some really cool stuff and we have a chance to build something interesting that you can always be proud of. We can’t offer much in the way of salary right now. We can offer you a block of stock. The reality is, the stock may never be worth anything. If we get sufficient funding, we are going to raise salaries to market rates. I’m looking forward to this, since I’m making the same (or less) than you would be.
I think this is a great way to explain the situation and present both the increased equity stake and the lower salary. Only addition I would make is that the day to day job will likely offer more ownership opportunities than a role that started at market rates.