Thresh's lantern. Click it. I main Thresh, and I can't tell you how many times I've had to explain to the ADC I'm supporting that they can teleport to me when they click my lantern. In fact, I think it was every time.
There have been hundreds of times I could have easily saved someone, but failed because they didn't click my lantern.
Just, study up on how Thresh works if you main ADC. Thresh is useless unless the ADC knows how he works.