Can Pass Without Trace affect anyone within 30 ft. at any time while the spell is active, or only those who were within 30 ft. when it was first cast?

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?