Karatomo Breakdown:English gloss: karaoke friend
constructed from: kara+tomo
kara: from カラオケ (karaoke, which further breaks down to kara (empty) and oke (orchestra)
Keep in mind karatomo could also mean "empty friend," but it doesn't ;-).
There is another couple of terms related to this. There is ヒトカラ (hitokara, going to karaoke not with friends, but hitori [alone]). There is also カラコン (karakon, a karaoke party). Do you recognize the kon part of that last term? It's related to a word I used last time, gōkon. Here is a karakon vid:
シャカリキボーイズ 投稿者 ROKUSASU0408
Yep it's the guys from yesterday. Here is another one (share my cheesy addiction):