I don't understand exactly what you are trying to accomplish but let's try.
If my assumptions are correct you are trying to customize EPESI so that you have 2 levels of users:
- Sales Representatives and
- admins - let's call them Managers.
Sales Representatives are limited to see only customers of their own (assigned) and Managers can see all customers. In addition Sales Reps can add new customers, edit existing ones.
You can do it in EPESI following these steps:
- Install module called Account Manager: Menu->Admin-> EPESI Store:

- Once installed assign your Sales Rep to a company - this is one to one relationship: One Company -> One Account Manager (Sales Rep). You can even rename using translation module Account Manager to Sales Representative.
- Create new Aceess Level called Sales Representative. To do this open Menu->Admin->Common Data. Open Array called Contacts and Access. Add new array (access level name) with values:
key: salesrep, Value: Sales Representative:

Open your employees = contacts listed under your company, and assign all your sales reps to this newly created group under Login Panel, field: Access:

Now you need to use Advanced Record Permission in Record Browser:

and create the following rules:

Rule: View
Access Level: Sales Representative
Criteria: Account Manager = (equal) User Contact (meaning logged in user)
You can also hide here some fields if you want.
You need to do the same for rules: Add and Edit
Do not let your Sales Rep delete any records or you can let them delete records that they created if you wish.
This covers Companies, but you need to do the same for View, Edit, Add, Delete for Contacts. Here the rule will be a little bit more complex:

Action: View
Clearance required: Access: Sales Representatives
Criteria: Company Name - equal - records with Account Manager set to - User Contact.
Repeat this process for Add, Edit and Delete.
This should do the trick. Please let us know if this guide is clear enough for you and if you were able to configure your EPESI installation to your liking.