You have to distinguish Contacts/Companies groups and user access levels (that help with access restrictions).
Let X be a navigation steps 😉
[X] Menu -> Admin -> Common data
Contacts/Companies:
[X] -> Companies_Groups, Contacts_Groups
User access:
[X] -> Contacts -> Access
I hope you'll get it 🙂
key is the important thing in the commondata. You must not change it after you've used it somewhere in the system. Value can be changed later.