CSB 3 John 1:14: Peace to you. The friends send you greetings. Greet the friends by name.
mGNT 3 John 1:14: εἰρήνη σοι ἀσπάζονταί σε οἱ φίλοι ἀσπάζου τοὺς φίλους κατ’ ὄνομα
What does he mean by "the friends"?
Generally the brethren are referred to as "brothers" or "the saints".
The letter seems to be written from an elder to a guy with a big house who hosts the local assembly and hosts travelling preachers and stuff:
NIV 3 John
1This letter is from John, the elder.a
I am writing to Gaius, my dear friend, whom I love in the truth.
2Dear friend, I hope all is well with you and that you are as healthy in body as you are strong in spirit. 3Some of the traveling teachersb ["the brothers"] recently returned and made me very happy by telling me about your faithfulness and that you are living according to the truth. 4I could have no greater joy than to hear that my children are following the truth.
Caring for the Lord’s Workers
5Dear friend, you are being faithful to God when you care for the traveling teachers who pass through, even though they are strangers to you. 6They have told the church here of your loving friendship. Please continue providing for such teachers in a manner that pleases God. 7For they are traveling for the Lord,c and they accept nothing from people who are not believers.d 8So we ourselves should support them so that we can be their partners as they teach the truth.
9I wrote to the church about this, but Diotrephes, who loves to be the leader, refuses to have anything to do with us. 10When I come, I will report some of the things he is doing and the evil accusations he is making against us. Not only does he refuse to welcome the traveling teachers, he also tells others not to help them. And when they do help, he puts them out of the church.
11Dear friend, don’t let this bad example influence you. Follow only what is good. Remember that those who do good prove that they are God’s children, and those who do evil prove that they do not know God.e
12Everyone speaks highly of Demetrius, as does the truth itself. We ourselves can say the same for him, and you know we speak the truth.
13I have much more to say to you, but I don’t want to write it with pen and ink. 14For I hope to see you soon, and then we will talk face to face.
 fPeace be with you.
Your friends here send you their greetings. Please give my personal greetings to each of our friends there.
Footnotes: a 1 Greek From the elder. b 3 Greek the brothers; also in verses 5 and 10. c 7a Greek They went out on behalf of the Name. d 7b Greek from Gentiles. e 11 Greek they have not seen God. f 15 Some English translations combine verses 14 and 15 into verse 14.
Is it possible that "the friends" references traveling preachers from the elder's assembly there as guests? Perhaps a Christian idiom? Might it be saying:
"The friends (your former guests from us) send you greetings. Greet the friends (travelers from us there as guests) by name (κατ’ ὄνομα)".
Supporting that idea is verse 3:
NIV 3 John 3 Some of the traveling teachersb [literally, "the brothers"] recently returned and made me very happy by telling me about your faithfulness and that you are living according to the truth.
Is that the right idea?
Please see this related question.