Question

He knew who were the right people to listen to and who to avoid (in infinitive sentence )

Answer 1

The infinitives in the given sentence are:

to listen

to avoid

He knew who were the right people  to listen to and who to avoid.

Answer 2

The infinitive in a sentence uses as something other than verb in a sentence and it comes with a preposition To.

So to write an infinite verb you have to write like To+ Infinite verb form.  

Following this rule in the sentence mentioned in the question we can find to infinite verbs namely ‘to listen’ and ‘to avoid’.

They both are verbs but here is used in a different way.