Quick answer: Yes, it does add, however stealth is not an invisibility cloak, but rather a proficiency in utilizing your surroundings through observation and strategy.
Looking at one, if not the best groups that could use stealth, the ninja, we can learn a lot how stealth works. They remembered to both mask themselves from all 5 senses while keeping strategy over strategy to stay one step ahead. Here's a few keys they focused on:
Observe your surroundings
Like V2Blast mentioned, hiding in a dark corner won't mask your sound, breath, or smell, and it won't guarantee that you're not seen either. With enemies knowing darkvision or having beast senses, it's more important to observe not only places to hide, but paths you can take to move, exit strategies to lose a tail, the look of the room (how dark and the pattern/color scheme) as well as keeping in mind the abilities of the enemies you're facing.
They had many tactics for this, from observing hiding places, paths they could take with many exit strategies, and how to avoid detection, or distract from pursuit.
Blend, don't hide.
As the prior section mentioned, hiding isn't the best strategy. With darkvision, observance proficiencies and senses of smell and hearing in play, a bundle of black cloth in a dark brown corner does stick out like a sore thumb. Even combining the spell darkness won't help hide you when you're in a brightly lighted room... The focus is to be proficient in blending with your environment, knowing where you can almost merge with the surroundings and when to leave.
Note: This tactic can also be related to abilities ninjas used like the tanuki no jutsu (or racoon perch) where staying in a high place (above 45 degrees from vision line) is often a place where people don't look. And when you do come down, assuring that it's behind the enemies' back. Wherever you go, Stealth is the mastery of knowing how to find your path from point A to point B with minimalizing chances to hide when possible or creating opportunities to do so when needed.
Know your advantages
Stealth is, in a way, the opposite of you being able to slip past guards on your own. As mentioned before, it's an ability to know how to use your environment, enemy behaviors, etc. to your advantage. That said, your team can also create an opening by having a mage create a sound down a hall to attack their eyes away from you as you whip by like the wind, or know how to utilize both darkness and light to your advantage. For instance, say you were trying to sneak past a guard and the only hiding place you had to hide was in front of a large bonfire. Rather than thinking you're screwed, high stealth could allow you to successfully cast the light cantrip to blend into the flames until they look away.
You might also be the one to use stealth, but note that this ability will increase if your team helps you out.
In short: High stealth will allow you to sneak through rooms unnoticed, but not because you're invisible. Rather, it is a reflection on how well you mastered the observation of paths, the timing of your travel, and disarming of potential obstacles that make up the journey as you sneak. For that reason, having a guarantee in this proficiency isn't ridiculous, but achievable through practice and training.