"peer WHO or peer THAT"?

"I would rather learn among peers THAT are friendly"
or
"I would rather learn among peers WHO are friendly"?

Also, is it bad if at the end of a 1st person (not very formal) essay, I incorporate a lesson that I learn and end by saying something about the opportunies "we" (not I) have to apply this lesson?

People are "who," everything else is "that." Peers are people, so "who" is the correct choice.

As to the second point, it depends. You'd have to post the essay for me to say for sure. Ask yourself whether or not the shift is done for a reason, and whether or not the reader is likely to go along with being included in your example.

"I would rather learn among friendly peers" works too