It's based on the S&P's eligibility criteria listed on pg.5 : https://us.spindices.com/documents/meth ... ndices.pdf
Tesla currently meets 4 of the 5 criteria except "the sum of the most recent four consecutive quarters’ Generally Accepted Accounting Principles ("GAAP") earnings (net income excluding discontinued operations) should be positive as should the most recent quarter".
This is going to be tough for Tesla as far as i know (limited personal knowledge of this company) they've only had 1 quarter of the last 8 that resulted in GAAP profits. I have no idea if the new Model 3 will really take this company to sustainable profitability given how R&D intensive they are but if and when they are close to be added to the S&P I'm going to be a buyer. Just need to follow the company's ER's going forward which won't be too difficult given their exposure on the news circuit anyways.