Do the beneficiaries of the Pass Without Trace spell change depending on who is in the 30-foot radius throughout the duration?

Question

The pass without trace spell description reads:

A veil of shadows and silence radiates from you, masking you and your companions from detection. For the duration, each creature you choose within 30 feet of you (including you) has a +10 bonus to Dexterity (Stealth) checks and can't be tracked except by magical means. A creature that receives this bonus leaves behind no tracks or other traces of its passage.

Does this mean that an ally who moves within 30 feet of the caster after the spell has been cast can begin to benefit from the spell? Does an ally who leaves the 30-foot radius while the spell is in effect lose the spell's benefits?

Answer 1

As you point out, the pass without trace spell description is somewhat ambiguously phrased regarding both points. In addition, the Sage Advice Compendium does not address such edge-cases regarding the spell. However, rules designer Jeremy Crawford has unofficially clarified both cases on Twitter.

It benefits only those who stay within 30 feet of the caster

Crawford clarified whether the benefit lasts even if those targeted move out of range in a January 2016 tweet:

Targets of Pass w/o Trace must remain within 30 ft. of caster in order to maintain benefit?

Pass without trace is meant to benefit you only while you're within the spell's radius.

This seems like a reasonable interpretation of the spell's description, since the way it's said to work is that "A veil of shadows and silence radiates from you". It only extends a certain distance from the caster.

(How the Stealth check is handled once a target leaves the 30-foot radius is ambiguous, but I think the simplest and most reasonable way to handle it is simply to subtract the added +10 bonus from their previous Stealth check - since otherwise your Stealth check is kept until you're discovered or you stop hiding.)

You can change who benefits from it throughout the duration

Crawford addressed a question about the spell's targeting throughout the duration in a July 2016 tweet:

pass without trace: can different targets be selected each round for the duration or are only the initial targets afffected?

Pass without Trace: you can choose different targets throughout the duration.

Again, this seems to be a logical possible interpretation of the spell description: "For the duration, each creature you choose within 30 feet of you (including you) [...]". One way to interpret this is as Crawford has - that it's not a one-time choice, but a repeated choice you can continue to make/change throughout the duration.

Answer 2

Notice that the spell has a range of "self", not a radius. This means that while the spell is active, the caster has this active effect. That effect works on creatures who are within thirty feet of you when they make their checks. There is no provision for it to persist for creatures who start within that distance but end up out of it (either because they leave or you walk away).

There is a bit of unclear interaction with the stealth rules. See Is stealth roll required each round in order to stay hidden?, the answer to which is "no" — normally you make one stealth roll and don't need to reroll unless you are discovered or decide to stop hiding. A reasonable interpretation might be that if you leave the "veil of shadows and silence", this is like leaving hiding and a new roll is required (this time of course without the benefit of the spell). But I've also played with DMs who ran things with the same roll persisting. That, of course, makes the spell slightly more powerful.