"Report X for not grouping"

If by not grouping, you're running out by yourself and get caught by 4/5 of the enemy team - essentially feeding.. then maybe?

I don't know, I have never reported a player for not grouping, unless it was extremely obvious he was doing it to feed but have it not look intentional. Been in a game where a teammate wanted to surrender. When we didn't forfeit, he roamed lanes alone in full vision of the enemy team while the rest of our team was together pushing objectives. He died over and over.

PS it is rude to ask someone a question and then tell them to "STFU" before they can even answer it.

asking to report someone is against the rules yo

Asking for reports is a reportable offense in-and-of itself.