How would you value companies that have negative earnings?

It would definitely help to find out why the company is experiencing negative earnings in the first place. For example, a company experiencing negative earnings due to early stage growth is very different from a company that is on a decline in a mature industry.

For early stage growth companies, using industry specific multiples might help. For example, in Biotech companies, a multiple can be used on what stage of FDA approval it is at. For tech companies, it could be number of users, number of downloads etc

For companies that are on decline, liquidation value might be a better approach, but this method is flawed as well.

If I were you, I might look at potential growth opportunities and maybe a qualitative analysis of the business models.