Are all users you created under the same main company? Or just the first ones you can use are within the same company? Check this first.
The best way to create a user account for your employee is to clone your contact first. Change first and last name of course and e-mail address etc. Create new login (user) at the same time just don't assign any roles in admin section (choices are no access, admin access and superadmin levels - you want no access), and under Access Level (this puts a user under the group of users with strictly assigned access rights to all records and you control it) choose Employee.
Employees can create and edit and delete their own records, but can not delete records created by others.
Managers can do more - they can delete all records, and edit of course.
This way you will create your first user and then you can clone him - access rights will be already preset.
An Employee is a contact within your Company with a login (username).
