Customer profiles will respect your account’s visibility rules by default. This means that the rules that govern what users can see, and cannot see, in your account will not be changed simply by converting a company to a customer.
Adding a user as a collaborator will give them visibility
However, it is possible to share visibility with other users on your account by adding them as ‘collaborators’ on your milestones. When you add someone as a collaborator, this will grant them visibility to the customer on the Company tab list view and will enable them to look at the profile.
When you add someone as a collaborator on a milestone and grant them visibility, they will have the ability to see all past activity on the record.
You can add a collaborator by first choosing the option to add a new milestone. 
You will then need to populate all fields indicated as required with the '*'. You can add multiple collaborators by clicking into the 'Collaborators' field and choosing user names from the dropdown. A reminder for this milestone can be created at this time to be sent to each collaborator by selecting the box for 'Send due date reminder'.
Removing a user will remove their visibility
There are two ways that you can remove a user’s visibility to a customer.
- If you remove someone as a collaborator on a milestone, they will lose visibility to the customer.
- If you convert the customer back to a company, then all collaborators who otherwise would not have visibility to the company will lose visibility.
Also note that if you revert the customer back to a company, and decide to convert it back to a customer, all the collaborators previously will not be available anymore. You need to reassign the collaborators again.