You can manage your employees' access to clients' accounts in two ways.
A) If you only want to allow/deny access to one account:
Here you can allow/deny access for any employee for this particular account.
B) Should you want to allow or deny access to multiple accounts:
Then simply select all the accounts that are relevant to the employee, as well as leaving open those accounts which are irrelevant.
These settings will be in place for the time being, but should you ever want to change them just run through this procedure again.