There was a choice and these people would just not see the salaries list and participate in salary raise process. But we went all in, everyone was ok with revealing their salaries.

That suggests either a very like-minded company (with 25 people very likely, with larger sizes less so) or tremendous amount of peer pressure. After all, if out of 25 people 24 decide to share, and 25th wants to work in this company, they don't really have a choice, do they? That person has to work with all these people after it.

So there was the implicit threat that if you didn't join, you wouldn't be eligible for raises? Also, how could you enforce them not seeing the list? It's illegal for an employer to deter the discussion of compensation amongst employees.

They would get raises in an old way, from CEO. We are based in Poland, and as far as I know there is nothing illigal about it. Someone mentioned shoulder surfing - I believe people who would not join, would be decent enough to play by the rules and not do this.

These people may not have access to the document, but surely in a small company they would figure out some of the salaries either through the grapevine or shoulder surfing?

There are three companion articles to this one which answer questions like this. The answer to your question from the "how" article: "Not joining would mean that others won’t know about that person’s salary and that person wouldn’t have access to the salary list."