Prove that in any group of 6 people there are always at least 3 people who either all know one-another or all are strangers to one-another.

Hint: Use the pigeonhole principle.

I don't see how this applies to the pigeonhole principle because I keep imagining a group of 4 strangers, and then 2 friends. This would be 6 total but against what the proof is asking. Maybe I don't understand the proof in question...