"Between existing records"?
Move away from the spreadsheet mentality. Access is not a spreadsheet.

The answer is you don't. Access stores records in no particular order,
so any new record is added in no particular order.
The order that you see is in accord with what ever sort order has been
set.

You wouldn't, and you're misunderstanding how tables work.

A table is an unordered "heap" of data. It has no order; there is no such
concept as "between". It's NOT a spreadsheet, even though it looks like one!

If you want to see records in a particular order, you must - no options! - use
a Query based on the table and sorting it in a particular order. You can have
Access create a hidden query for you by choosing Records... Sort in table
datasheet view, but that does not affect how the records are stored, only how
they are displayed.

You should not be using table datasheet view to enter or edit data in any
case: tables are for data storage, you should create a Form (which can be
based on a sorted query if you wish) for data entry and editing.

            John W. Vinson [MVP]