What do you do when your team refuses to group?

This is becoming quiet the problem in every game I seem to lose, we'll be winning (usually by me getting map pressure now I know how to play properly haha) but all of a sudden, somebody walks off on their own solo and dies....again..and again...When I say we should group and win (which 90% we'd win in this case) somebody will usually randomly split push or just walk off again, and we'll lose because they get caught and get collapsed on in a 4v5. So how do you prevent this or somehow get the to group? Thanks for any advice :) This is in ranked and normal by the way. (and yes I'm low elo but still, this is becoming annoying when I'm like 14-3 and happens again and again despite using my power)
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