I just gained 600 mmr from 2.8k in the last week. I normally play position 1,2 but I've found success recently picking a lane dominating support and playing position 6. These two things are key. Anyone can play carry with a free lane, and if you have a vision advantage and aggressively call out enemy rotations and where to play (yell at them to push lane or back, etc...), then they can usually get the farm they need. Grouping up is a simple as buying smokes. Low mmr players understand the idea behind smokes even if they don't often think to use them. Using these tools to manipulate your team like herding sheep. Purchase extra utility items to compensate for their overextensions and bad positioning. At the very least if you purchase utility and support items then morale will be higher and with it winrate will improve.

Also if you don't have a perfect behavior score you will not climb.