Keep track of teammates fatigue.

First let me introduce you to the concept of fatigue. It's an umbrella term to regroup anything that could make a champion weaker the his optimal state in game after a fresh back to the shop. It include.

1. HP bar. Duh !
2. Mana bar. again duh !
3. Cooldown of abilities. Obviously for long one like ults. But also for shorter one on short term. To be clear the best combo of your teanmate might be to burst w-q-e in that order but the cooldown of the 3 spells are likely not to be the same. So after the initial burst, as long as he stand in fight, the 3 spells will likely never align again pretty well until he break for a few seconds.
4. Cooldown on summoners spells.
5. Gold. While in role play, gold is actualy a buff you didn't yet bought. For all purpose here it's just something that make you weaker then your opponent who had time to shop.
6. Stacks in some cases.

First thing to observe is that, while many champions have many relation with all of those. At the end of the day, even Mundo gather enough gold do fall behind. You can't recover from gold.

Being aware of this and keeping a mental note on every of your teammates will greatly improve your win rate. First, try to tune your fatigue level with your teamates. If you want to group but 3 of your teammate have 3k gold they will just all back on you. Or worst try to cover your aggressive back and lose the next teamfight. instead, head toward small objectives you can chew yourself (a wave of minions, a few jungle camps) and wait for them to back before you go for the next play.

Reversely, if you're the last one standing. Even if you technically have health and could keep going. The best might be to back right away and tune your fatigue with the others. Or if all of your team just join mid for a fight, it doesn't really matter if YOU personally could end a bloodthirster from the bf sword and scepter you already have. It's only 800 gold ! You would burn the prime time for a good team fight for a selfish, half efficient shop. When you will be back from it, your team might still have hp and mana right, but they will have fatigue one way or another from holding a 4v5.

Lastly, but most importantly in low elo !! Always assume the second team fight in a row to be an instant lost. Unless you already won the last 5 of them and you are ready to end. Even if you do outskill them to that point, it will still be easier if you shop and refresh yourself. You will be even closer to the victory.

tl,dr; Keep track of when your teammates shops and be aware of when they might want to and when they'd prefer to make a big play. Adjust with your team.