Direct buses are not available till auli... u have to board a bus till joshimath from dehradun and then take a ropeway to auli... buses start from 3am from dehradun to joshimath.
Since there are no direct buses you've to take bus to Joshimath, which is some 16km away from auli, you can take bus from Joshimath to auli or opt for a shared taxi