An edit to Thieves Cant

[Witty Username]

Back to the OP though -

How would this ability work in combat? Can the rogue hide everyone with a bonus action (assuming conditions were right, like dim light)? Light and sight are not well defined in 5e - to the rogue's detriment, IMO - and this would compound on that in a big way.

At the moment, the intentions would be that this would be functional in combat, and the rogue would be able to hide party members as a bonus action with cunning action if they so choose and if conditions allow.

The DM determines when hiding is possible, that is the rules we got. I don't think we are going to solve that argument here.

As for the initial idea, as it does deserve a real answer. I am not sure I love the idea of everyone sharing the Rogue's roll, you can get in to some verisimilitude issues. But I could see a bonus like +5 up to the result of the Rogue's roll. Or maybe the Rogue share's their proficiency for those lacking it.

Sharing the roll is mostly to smooth over full party stealth, without just making the bonus bonkers like pass without trace does. One roll for the party ish, at least as I thought of it from a verisimilitude sense was like Zuko (blue spirit) escorting Aang out of jail from that early episode of Avatar, one could take issue with the effectiveness of it but the root idea is there.

The lighter version would probably be Adding prof bonus to allies and or providing advantage (stealth help), but that runs the risk of the one dude in heavy armor ruining the op for everybody.