Hey, welcome to Discourse Meta!
The main way to manage permissions in Discourse involves “groups” and “categories”. Basically, you’d create a group for these specific users (e.g. trial_customers), and a category for the topics where you want to limit access (e.g. “Customer Support” or whatever). Then you edit that category’s permissions as needed for those users.
Similarly you can do the inverse e.g. for staff only categories, or ones where only paying customers should have access but not trial customers.
See here for detail on how this works: