The best way to address this is to pair the person with a mentor who has been with the company a while and has good relationships with many of the fellow employees.
This doesn't have to be someone in a similar role. This person can give introductions to people in the department, company, and people they might be working with in different departments.
The person should know the personalities involved and the goings on. The person should be able to give pointers like.
Don't approach Bob on Thursdays, it's his busiest day.
Sue is a great go-to person. She may not have the answer, but can send you to the person who does.
In other words, a person who has been around a while, has good relationships with people, and is involved in creating back-channels and knows who to go to for what.
You want to make this as unofficial as possible and it may need to rotate among several people depending on workload as you don't want this to become a full-time job for anyone as that will put them in an official role and thus become part of the "official culture" as opposed to one who is effective of navigating the unofficial one.