How many times do you have to yell group, before your team starts to listen?

If they dont follow you, you should follow them.

Only once if you have something more convincing to say than "GROUP UP ALL MID"

the really hard part is convincing cowards to splitpush lol "BUT IM PLAYING TEEMO!"

Depends on your tier. In bronze, until you turn blue (most of the time that is, there are exceptions), in silver, they might listen after a few times, but will take a while. In high plat, it's magic, you call baron, everyone stops what they're doing (within reason) and goes baron, no "I'll just cs 2 more waves then go", no "I need to b to finish item wait for me"... same idea for grouping. First time I played with people of that ELO, the difference felt amazing. So yeah, ELO dependent. For a reason.

Try building consensus as opposed to yelling.

well considering people are from all over the north america the chances that they arent gonna hear you yelling

If they aren't listening to your shot calling, it's because you haven't given them a reason to. Focus on your personal game and if they see you doing well, they're more likely to listen to you as a leader instead of reading you as a control freak trying to salvage a losing battle.

At that point if they don't listen, it's better to stick with them instead and lead from the rear, since splitting up your team because you can't get them to listen to your calls is how you lose games. It's all about give and take really.