"There were people there that I didn't even know who were"

Solution 1:

It's often helpful, when sentences are getting messy like this, to replace a complicated phrase-verb with a more straightforward verb. The phrase-verb that's causing the problem is "(to) know who (someone) is" - we can replace it with the simpler verb "(to) identify (someone)". So our sentence becomes: "there were people there whom I couldn't even identify".

Alternately, because your basic goal is to describe the people, you can toss out the verb construction all together and just use an adjective! Replace "who I didn't know who they were" with "who were unidentifiable". Or, for something a little more casual, perhaps "some of the people there were a total mystery to me".