An Agile team should have automation testing knowledge from the start, the team should be cross-functional. Personally I think a test in each layer of the test pyramid should be defined in the definition of done for each story (feature) that is done.
Creating UI tests as an after thought will result in harder testable software. During development it is easy and cheap to change the UI a bit to make it testable from the start. Don't have testers struggle months after the development is done.
Make sure you write a couple of UI tests each iteration. This will make it easier to write end-2-end test for new functionality and defects in a later stage, certainly for defects that are hard to test in unit-tests.
Also your automation testers will not have the domain knowledge to write good tests, because they will join the project later and did not grow with the product.
Keep a good balance between UI and Unit-tests, UI tests are harder to maintain and with a constant changing UI it might even get frustrating to update them. Make sure UI tests are written as maintainable as possible, have the team use patterns like PageObjects.