Yes! It it possible to manage access control to both Groups and Cases individually.
By clicking the "Access"-button, you will see a list of all users in your organisation - and can add or remove them directly from that panel. You can also elevate or demote their access roles (see below).
If you give a user access to Groups, they cannot necessarily see the Cases, but if you give them access to a Case, they will automatically get access to the Group that the Case is connected to.
There are two types of user access roles:
Administrators can manage access in Groups and Cases and create template content in Groups
Members can see everything in Groups, but not edit. Members can create new Cases and content, timelines, whiteboards and deadlines in such Cases.