At series C you are not likely to get a significant enough position to make it worthwhile. If they do indeed have revenue then they should be able to pay you a decent salary.

This is the way I look at it:

A 50% chance to make $100K in 5 years with an interest rate of 5% is worth:

($100K * 0.5) / (1.05 ^ 5) = $39K

And if you have credit card debt then you should be discounting at something closer to 15%.