It seems you have read-in both the Employers table and the Employees table in DataTable objects and wish to filter the Employees table based on the Employer ID from the Employers table the user selects. You can do this easily by creting a DataView object based on the Employees DataTable like this.
DataView employeeView = new DataView(employeeTable);
Now suppose the Employer ID has changed and you wish to filter employees based on it. Assume that the Employer ID is numeric. You would do it like this:
// Assume we have ID of the selected Employer in a variable employerID
employeeView.RowFilter = employerID;
This would filter the employeeView DataView so that it would contain only those employees whose id is equal to employerID.
If you've bound a control, a DataGrid or ComboBox for example, to the employeeView, the control would auto-update itself to reflect whatever data is contained in the employeeView. In other words, the bound control would contain the filtered employees (because the DataView has been filtered).
I hope this helps.