How far ahead do you need to be to stealth without penalty?

[Tony Vargas]

Technically, you could be stealthy surrounded by PC bards that are all playing bagpipes assuming you have sufficient cover.

To be clear, I clicked laugh for this bit.

The rest is dead-on, deserving of XP, but I can't do both...

So the real question is how far away before the enemy notices your group. Which in my campaign is a too varied to have a set number. Basically far enough ahead that you can't detect them unless they call out or far enough ahead that even if they detect your group they won't notice you. But it depends on what your goal is. Start combat hidden? Twenty feet may be enough. Surprise your enemy? Depends on terrain, ambient noise, are they watching. Could be twenty feet, could be two hundred.

In other words, ask your DM, because their opinion is the only one that matters.

(And, if you are the DM, even if you're unsure of a ruling, present it with confidence and don't be baited into debating it, let alone changing it.)

People come here asking for advice, sometimes there is no concrete answer because D&D is not a board game.

To be fair, you can get a concrete answer in the context of an RPG, as well, if the RPG in question happens to have a clearly stated rule that applies. 5e has fewer clearly-stated rules not because it's an RPG, but because it's written in natural language that often requires DM interpretation, and employs mechanics that explicitly call for DM judgement quite frequently.

A board game could lack concrete answers to rule questions, too, resulting in varied 'house rules' - it's just more likely it'd be called 'badly written' and less likely said ambiguity would be perceived as a feature.

 

[aco175]

I typically make the rest of the party a full round behind and they come in at the edge of the board on the start of their next round. It is not the most realistic, but few situations have come up where it needs to be better and the rest of the players want to get in and help if something comes up.