Intentionally losing as a 5 man group

This is probably a stupid question, but I would really like some clarification before I do anything stupid.

I made a smurf to play with low elo friends, they're between low silver and low gold, but my smurf account is climbing much higher and I'm already gold 1. I know that if I get to plat, I wont be able to play with most of my friends, so that leaves me with 3 options. First, I could dodge every promo match, which as I understand is entirely legal and fair, but takes a lot of time since the penalty for dodging would mean 30+ minute waits. Second, I could just play normals with them, which only 1 other friend actually enjoys playing, so that's not really a good option.

Lastly, and this is my question, is playing with as a group of 5 and losing on purpose legal? I know that trolling other people's games and making other people lose is bannable, obviously, but what if all 5 people in the premade group want to lose? Thats not trolling anyone, so is it okay?